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CONVERSATIONS  ON  CHEMISTRY. 

FIRST  STEPS  IN  CHEMISTRY. 

Part  I. 

General  Chemistry. 

BY 

Prof.  W.  OSTWALD. 

AUTHORIZED  TRANSLATION 

BY 

ELIZABETH  CATHERINE  RAMSAY. 
i2mo,  viii  -|-  250  pages,  46  figures.     Cloth,  $1.50. 


CONVERSATIONS  ON  CHEMISTRY 


FIRST  STEPS  IN  CHEMISTET 


BY 

W.  OSTWALD 

Professor  of  Chemistry  in  the  University  of  Leipzig 


ATTTHOmZED    TRANSLATION 

BY 

ELIZABETH  CATHERINE  RAMSAY 


Part  I 
GENERAL  CHEMISTRY 


FIRST  EDITION,  CORRECTED 
THIRD   THOUSAND 


NEW   YORK 

JOHN  WILEY  &  SONS 

London:  CHAPMAN  &  HALL,   Limited 

1911 


Copyriglit,  1905, 

BY 

ELIZABETH  CATHERINE  RAMSAY. 
Entered  at  Stationers'  Hall. 


THF  SCIENTIFIC  PRESS 

nOBERT   ORUMMOND   AND   COMPANY 

BROOKLYN,    N.   Y. 


-.  .vA^.*iA*.'W»    c. ■ .  i^-V. 


AUTHOR'S    PREFACE. 


The  causes  which  led  me  to  write  this  work  lie  partly 
in  the  past,  partly  in  the  future.  The  former  spring 
from  the  feeling  of  thankfulness  with  which  I  even  now 
regard  the  "Schule  der  Chemie"  of  Stockhardt,  whose 
memory  still  lingers  among  us.  By  a  stroke  of  good 
fortune  this  excellent  work  was  the  first  text-book  of 
chemistry  which  was  placed  in  my  hands,  and  it  influ- 
enced the  whole  of  my  subsequent  activity  in  science. 
Owing  to  the  carefully  thought-out  directness  in  repre- 
senting the  facts  to  the  pupil,  the  skill  in  selecting  experi- 
ments suitable  to  the  physical  and  mental  powers  of 
the  beginner,  I  have  never  lost  touch  with  experiment, 
although  I  have  been  chiefly  occupied  with  general 
questions  of  science.  The  request  of  the  publishers,  who 
used  to  issue  this  work,  that  I  should  write  a  modern 
Stockhardt,  was  both  an  honour  and  an  opportunity  of 
paying  off  an  old  debt  of  thankfulness. 

So  much  for  the  past. 

As  regards  the  future,  chemistry  has  undergone  during 
the  past  century  an  enormous  development,  in  which  Ger- 
many has  played  an  important  part.  Chemical  science  in 
Germany  has  been  furthered  by  the  work  of  thousands 
of  diHgent  hands  and  greatly  aided  by  educational  insti- 
tutions which  have  become  a  pattern  for  the  whole  world, 
which  have  brought  about  a  constant  interchange  between 


IV  AUTHOR'S  PREFACE. 

science  and  its  applications,  and  which  have  given  an 
uninterrupted  proof  of  a  continued  heahhy  existence. 
It  was  almost  entirely  organic  chemistry  which  developed 
in  the  direction  of  the  discovery  of  new  bodies  and  their 
systematic  arrangement ;  and  even  to  this  day,  by  far  the 
majority  of  young  chemists,  after  hurrying  through  a  short 
course  of  analysis,  are  trained  in  these  methods. 

But  hasty  progress  has  its  dangers,  and  it  is  the  duty 
of  every  man  who  tries  to  look  into  the  future  to  give 
a  timely  word  of  warning;  for  inorganic  chemistry  was 
a  science  before  organic  chemistry  was  thought  of;  more- 
over, the  processes  of  inorganic  chemistry  form  the  basis 
of  chemical  technology,  on  which  that  of  organic  com- 
pounds is  a  superstructure. 

The  cry  was  first  raised  in  manufacturing  circles  that 
the  young  chemist  trained  exclusively  in  organic  chem- 
istry was  unfit  to  cope  with  the  solution  of  general  prob- 
lems; with  that  reciprocity  between  science  and  tech- 
nology so  characteristic  of  the  German  race,  the  teachers 
of  our  science  have  at  once  grappled  with  the  problem. 

Among  the  many  proposals  which  have  been  made  to 
escape,  in  good  time,  the  pressing  danger  of  chemical 
onesidedness,  none  appears  to  me  more  suitable  than 
the  encouragement  of  the  growth  which  has  developed 
upon  the  soil  of  science  during  the  last  ten  years.  I  refer 
to  general  and  physical  chemistry.  It  deals  with  ques- 
tions which  he  at  the  base  of  organic  and  inorganic,  of 
pure  and  applied  chemistry;  it  forms  a  foundation  for 
all  real  chemical  education,  and  must  be  regarded  as 
lying  at  the  root  of  all  chemical  teaching,  especially  in 
its  earlier  stages. 

By  writing  a  series  of  text-books  dealing  with  different 
stages    of  the  subject  I  have  tried  to  bring  about  the 


AUTHOR'S  PREFACE.  V 

knowledge  of  these  principles  as  they  at  present  exist, 
first  among  my  colleagues  in  science,  and  next  among 
students  of  chemistry. 

The  necessity  of  repeatedly  revising  the  matter  of 
these  books,  as  well  as  daily  experience  in  teaching, 
led  to  my  early  conviction  that  the  very  first  steps  of  a 
young  pupil  must  point  in  this  direction;  I  also  gained 
assurance  that  such  an  introduction  was  possible,  and 
this  book  is  the  result  of  my  efforts. 

I  must  not  omit  to  mention  that  it  forms  the  first 
introductory  volume,  and  that  it  will  be  followed  as  soon 
as  possible  by  a  second  of  about  equal  length,  in  which 
the  system  will  be  more  developed. 

I  have  chosen  the  form  of  dialogue,  because  after 
several  attempts  it  appeared  to  me  the  most  suitable; 
moreover,  I  have  come  to  the  conclusion  that  it  occupies 
no  more  space  than  an  ordinary  description,  while  the 
impression  it  makes  is  much  more  penetrating  and 
lively.  I  venture  to  hope  that  it  will  be  found  that  it  is 
at  the  same  time  the  result  of  a  varied  experience  in 
teaching,  and  not  an  accidental  choice. 

W.    OSTWALD. 

Leipzig,  1903. 


CONTENTS, 


PAGB 

1.  Substances i 

2.  Properties 5 

3.  Substances  and  Mixtures 10 

4.  Solutions 16 

5.  Melting  and  Freezing 23 

6.  Boiling  and  Evaporation 28 

7.  Measuring 36 

8.  Density 46 

9.  Forms 54 

10.  Combustion 61 

11.  Oxygen 71 

12.  Compounds  and  Constituents 82 

13.  Elements 92 

14.  Light  Metals 103 

15.  Heavy  Metals 113 

16.  More  about  Oxygen 117 

17.  Hydrogen 129 

18.  Oxygen  and  Hydrogen 139 

19.  Water 152 

20.  Ice 163 

21.  Steam 171 

22.  Nitrogen 182 

23.  Air 188 

24.  Continuity  and  Exactness 200 

25.  The  Expansion  of  Air  by  Heat 208 

26.  The  Water  in  the  Air 218 

27.  Carbon 224 

28.  Carbon  Monoxide 234 

29.  Carbon  Dioxide 237 

30.  The  Sun 244 

vii 


CONVERSATIONS  IN  CHEMISTRY. 


1.  SUBSTANCES. 

Master.  To-day  we  commence  something  quite  new; 
you  shall  begin  to  learn  chemistry. 

Pupil.  What  is  chemistry  ? 

M.  Chemistry  is  a  branch  of  natural  science.  You 
have  already  learned  something  about  animals  and  plants 
and  know  that  the  study  of  animals  is  called  zoology,  and 
that  of  plants  botany. 

P.  Then  does  chemistry  teach  about  stones? 

M.  No,  that  is  called  mineralogy.  Mineralogy  is  not 
the  study  of  stones  alone,  but  of  many  other  things  which 
are  found  in  the  earth,  such  as  phosphorus,  gold,  or  coal. 
But  all  these  things,  too,  belong  to  chemistry.  And 
other  things,  like  sugar,  glass,  iron,  which  are  not  found 
in  the  earth,  but  are  artificially  obtained  from  other  sub- 
stances, are  also  the  subjects  of  chemistry.  Chemistry  is 
the  study  of  all  kinds  of  substances,  whether  artificial  or 
natural. 

P.  Then  does  chemistry  deal  with  trees  ? 

M.  No,  for  a  tree  is  not  a  substance. 

P.  But  it  is  wood,  and  wood  is  a  substance. 

M.  Yes,  but  a  tree  consists  of  more  than  wood,  for  its 
leaves  and  fruit  are  not  made  of  wood,  but  of  other  sub- 


2  CONyERSATIONS  ON  CHEMISTRY. 

stances.  All  such  substances  taken  alone  belong  to 
chemistry;  but  to  get  each  alone,  the  tree  must  be  de- 
stroyed. 

P.  But  what  do  you  mean  by  a  substance  ? 

M.  That  is  a  long  story.  Let  me  see  if  you  don't 
know  it  yourself,  though  you  can't  put  it  into  words. 
What  is  this? 

P.  I  think  it  is  sugar. 

M,  Why? 

P.  Well,  the  sugar  in  the  sugar-basin  looks  just  like  it. 
Let  me  taste  it — yes,  it's  sugar,  for  it's  sweet. 

M.  Do  you  know  another  way  by  which  you  can  tell 
sugar  ? 

P.  Yes,  it  makes  your  fingers  sticky;   so  does  this. 

M.  You  can  tell  sugar,  then,  when  some  one  puts  it  in 
your  hand  and  asks  you  if  it  is  sugar.  And  you  knew  it, 
first  by  its  appearance,  then  by  its  taste,  and  lastly  by  its 
stickiness.  These  signs  by  which  you  recognize  a  sub- 
stance are  called  ''properties";  you  know  sugar  by  its 
properties.  Sugar  is  a  substance;  one  can  tell  substances 
by  their  properties.  Do  you  think  you  could  use  all  the 
properties  of  a  substance  in  order  to  recognize  it  ? 

P.  Yes,  if  I  knew  them. 

M.  We  will  just  see.  Is  there  only  one  sort  of  sugar? 
No,  you  know  loaf  sugar,  which  is  in  large  lumps,  and 
sifted  sugar,  which  is  a  powder,  like  sand.  Both  are 
sugar,  because  when  you  pound  up  loaf  sugar  it  becomes 
like  sifted  sugar. 

P.  Yes:   then  they  are  both  the  same. 

M.  Both  are  the  same  substance,  sugar,  but  one  of  its 
properties  has  been  changed.  The  shape  of  a  thing  is 
also  one  of  its  properties;  if  you  like  you  can  change  its 
shape,  yet  the  stuff  of  which  it  consists  remains  the  same. 


SUBSTANCES.  3 

This  also  applies  to  quantity.  Whether  the  sugar-basin 
is  full  or  almost  empty,  what  is  in  it  is  always  sugar.  So 
you  see  form  and  quantity  do  not  belong  to  the  properties 
by  which  you  recognize  a  substance.  Is  sugar  hot  or 
cold? 

P.  I  don't  know;   it  may  be  either. 

M.  Quite  right.  So  neither  heat  nor  cold  is  a  property 
by  which  you  can  tell  a  substance. 

P.  No,  of  course  you  can't;  for  you  can  make  sugar  as 
coarse  or  as  fine,  or  as  hot  or  cold  as  you  wish. 

M.  Now  we  have  got  to  the  bottom  of  it.  Among  the 
properties  of  a  thing  there  are  some  which  cannot  be 
altered.  You  will  always  find  that  sugar  tastes  sweet, 
and  that  it  makes  your  fingers  sticky.  But  you  can 
change  its  size  and  form,  and  you  can  heat  it  if  you  like. 
Every  definite  substance  has  its  distinct  unchangeable 
properties,  and  a  thing  bears  the  name  of  this  substance 
when  it  has  these  fixed  unchangeable  properties,  quite 
independently  of  whether  it  is  warm  or  cold,  large  or 
small,  or  how  its  changeable  properties  may  vary. 

A  thing  has  often  another  name,  according  to  its  use 
or  its  shape,  different  from  that  of  the  substance  it  is  made 
of.    Then  it  is  said  to  consist  of  this  particular  substance. 

P.  I  don't  quite  understand  that. 

M.  What's  this?  what's  that? 

P.  A  knitting-needle  and  a  pair  of  scissors. 

M.  Are  knitting-needles  and  scissors  substances? 

P.  I'm  not  sure —    No,  I  think  not. 

M.  If  you  wish  to  know,  you  have  only  to  ask:  What 
does  the  thing  consist  of,  or  what  is  it  made  of?  Then 
you  generally  come  at  the  name  of  the  substance.  What 
are  knitting-needles  and  scissors  made  of? 

P.  Of  iron.     Then  is  iron  a  substance? 


4  CONVERSATIONS  ON  CHEMISTRY, 

M,  Certainly,  for  a  piece  of  iron  is  called  iron,  whether 
it  is  large  or  small,  hot  or  cold. 

P.  Then  paper  is  a  substance,  because  a  book  is  made 
of  paper,  and  wood  is  a  substance,  because  the  table  is 
made  of  wood,  and  bricks  are  a  substance,  because 
houses  are  made  of  bricks. 

M.  The  first  two  examples  are  right,  but  not  the  last. 
Does  a  brick  remain  a  brick  when  it  is  broken  up  and 
powdered?  No:  the  name  "brick"  is  given  to  a  thing 
that  has  a  definite  shape,  so  it  can't  be  a  stuff.  But  what 
are  bricks  made  of? 

P.  They're  made  of  clay. 

M.  Is  clay  a  substance  ? 

P.  Yes — no — yes,  it  is,  because  if  you  break  up  clay 
it  still  remains  clay. 

M.  Quite  right.  You  can  often  help  yourself  out  like 
that  when  you  are  in  doubt.  First  you  must  ask:  What 
is  the  thing  made  of?  And  when  you  have  answered 
that,  you  must  go  further,  and  ask:  Does  it  remain  the 
same  when  I  break  it  up?  and  if  you  can  say  Yes,  then 
it  is  a  substance. 

P.  Then  there  are  many,  many  different  kinds  of  sub- 
stances? 

M.  Yes,  certainly  there  are  many;  far  more  substances 
than  you  can  name.  And  chemistry  has  to  do  with  all 
such  substances. 

P.  Oh,  then  I  shall  never  be  able  to  learn  all  about 
chemistry — it's  hopeless.     I'd  rather  not  begin. 

M.  Do  you  know  the  forest  near  here  ? 

P.  Yes,  rather:  you  could  put  me  where  you  like  in  it, 
and  I  should  always  know  where  I  was. 

M.  But  you  don't  know  every  single  tree  in  it?  How 
can  you  help  being  lost  ? 


PROPERTIES,  S 

P.  But  I  know  the  paths. 

M.  Now,  look  here,  that  is  what  we  are  going  to  do 
with  chemistry.  We  will  not  learn  about  every  single 
substance  that  there  is,  but  we  will  learn  the  paths  which 
divide  up  the  countless  substances  into  different  groups, 
and  by  help  of  which  we  can  find  our  way  from  one  place 
to  another. 

When  you  know  the  principal  paths  you  will  know 
where  you  are  in  chemistry,  and  afterwards  you  can  leave 
the  chief  paths,  and  find  out  more  about  the  byways. 
And  you  will  see  that  learning  chemistry  is  just  as  good 
fun  as  walking  in  a  wood. 


2.  PROPERTIES. 

M,  Let  me  hear  what  you  learned  last  time. 

P.  Chemistry  is  the  study  of  substances,  and  sub- 
stances are  what  things  consist  of. 

M.  The  first  part  of  your  answer  is  right,  but  the 
second  is  not  quite  right.  A  piece  of  music  consists  of 
sounds;  but  are  sounds  substances? 

P.  Yes;  for  you  can  call  the  sounds  music  is  made  of, 
substances. 

M.  Yes,  in  a  figurative  sense  you  can.  But  in  the 
language  of  science  the  name  "substance"  is  limited 
to  things  that  have  weight. 

P.  What  right  has  any  one  to  limit  the  meaning  of  a 
name? 

M.  The  right  of  necessity.  In  the  language  of  ordinary 
life  people  are  not  generally  very  careful  of  the  meaning  of 
words,  as  you  showed  yourself  just  ngw.  In  science, 
however,  we  have  to  try  to  be  as  accurate  as  we  can, 


6  COhiyERSATlONS  ON  CHEMISTRY. 

and  that  is  why  we  have  to  give  every- day  words  an 
exact  and  accurate  meaning.  These  meanings  are  made 
as  like  as  possible  to  those  which  they  ordinarily  have, 
and  really  mean  the  same  thing  to  all  intents  and  pur- 
poses, only  the  boundary-line  of  use  and  meaning  is 
more  sharply  drawn. 

Most  things  which  are  generally  known  as  substances 
are  called  the  same  in  chemistry;  but  no  things  that 
have  no  weight.  Now  correct  the  last  part  of  your  sen- 
tence:   "Substances  are  everything"  .  .  . 

P.  A  substance  is  anything  of  which  a  weighable 
thing  consists.  Yes,  but  I  don't  know  yet  what  a  sub- 
stance really  is. 

M.  What  do  you  mean? 

P.  I  know  quite  well  what  things  to  call  substances, 
but  that  isn't  all.  It  doesn't  tell  me  any  more  than  I 
knew  before.  I  know  nothing  about  the  nature  of  a 
substance  yet. 

M.  How  should  you  know  it?  By  giving  a  word  a 
distinct  scientific  use,  or  defining  a  word,  nothing  more 
has  happened  than  that  I  drew  a  circle  round  it  so  as 
to  limit  the  meaning  of  the  word  within  certain  bounds. 
We  have  made  a  fence  round  our  forest;  but,  of  course, 
that  doesn't  teach  us  to  know  it.  As  you  learn  the  prop- 
erties of  the  various  substances,  you  will  also  learn  their 
nature,  and  that  will  give  you  plenty  to  do. 

P.  But  when  I  know  all  the  properties  of  a  substance, 
I'll  only  know — how  can  I  put  it? — the  outside  of  it. 
I  can't  get  through  to  its  inner  nature. 

M.  Don't  you  remember  that  there  are  different 
sorts  of  properties?     What  are  they? 

P.  You  mean  what  we  spoke  of  yesterday  ?  There  are 
changeable  and  unchangeable  properties. 


PROPERTIES.  7 

M.  And  which  help  you  to  recognize  the  substance? 

P.  The  unchangeable  ones. 

M.  There  now,  you've  found  what  you  want.  The 
unchangeable  properties  of  a  substance  can't  be  taken 
away;  when  they  aren't  there,  the  substance  isn't  there 
either.     These  properties  make  the  nature  of  the  substance. 

P.  But  that  is  only  its  properties.  What  I  want  to 
know  is :  What  lies  at  the  bottom  of  all  its  properties  ? 

M.  You  want  to  know  what  remains  when  you  think 
all  the  properties  of  a  substance  are  taken  away.  Now, 
just  think,  if  you  took  away  all  the  properties  of  a  piece 
of  sugar,  its  colour,  form,  hardness,  weight,  taste,  etc., 
what  would  remain? 

P.  I  don't  know. 

M.  Nothing  would  remain.  Because  it  is  only  by 
the  properties  I  can  tell  that  something  is  there;  if  no 
properties  are  present,  there  is  nothing  there  that  I  can 
speak  about.  You  must  get  rid  of  the  idea  that  there 
is  anything  higher  or  more  real  to  be  found  in  a  thing 
than  its  properties.  Long  ago,  when  science  was  little 
advanced,  people  thought  something  like  that,  and  there 
are  remains  of  it  in  ordinary  speech,  so  that  one  uncon- 
sciously gets  these  ideas  through  the  use  of  ordinary 
expressions.  But  once  you  recognize  this  error  you  can 
avoid  it. 

P.  I  see  you  are  quite  right,  but  I'm  afraid  it  will  take 
me  a  long  time  to  get  rid  of  the  other  idea. 

M.  You  will  be  convinced  when  you  have  barned  more 
chemistry  that  we  really  only  speak  of  the  properties 
of  a  stuff,  and  never  of  its  ''nature.''  And  you  will 
forget  your  mistake  later. — Anyhow,  this  talk  has  had 
its  use,  for  now  you  see  clearly  that  everything  depends 
on  our  determining  and  knowing  properties.     Tell  me 


8  CONyERSATIONS  ON  CHEMISTRY, 

some  properties  which  help  you  to  recognize  a  sub- 
stance. For  example,  what  is  the  difference  between 
silver,  gold,  and  copper? 

P.  The  colour;  silver  is  white,  gold  yellow,  and 
copper  red. 

M.  Does  colour  belong  to  the  changeable  or  to  the 
unchangeable  properties  of  a  substance? 

P.  Generally  to  the  unchangeable,  I  should  think. 

M.  Why  are  you  so  uncertain  about  it? 

P.  I  am  not  quite  sure:  the  colours  of  gold  and  silver 
are  unchangeable,  but  old  copper  doesn't  look  red,  but 
dark,  and  often  green. 

M.  Have  you  ever  looked  carefully  at  a  piece  of  copper 
that  has  become  green  ?  Is  the  copper  green  through  and 
through  ? 

P.  I  think  not;  no,  you  can  scratch  off  the  green,  and 
there  is  red  copper  underneath. 

M.  Quite  right;  and  the  green  is,  in  other  ways,  not 
like  copper;  it  is  not  tough  like  metal,  but  crumbly  like 
earth.  The  fact  is  that  another  substance,  green  in  colour, 
has  been  found  on  the  copper,  that  was  not  there  before, 
and  it  has  only  covered  up  the  red  copper,  just  as  the 
yellow  wood  of  the  window-frame  is  covered  with  white 
paint. 

P.  How  does  the  green  come  on  the  copper? 

M.  It  is  formed  from  the  copper :  you  will  learn  later 
exactly  how  it  comes.  At  first  we  will  go  back  to  the 
question  of  colours.  Now  we  must  take  colour  as  an 
unchangeable  property,  by  which  we  can  recognize  a 
substance.  Only  we  must  take  care  not  to  mistake  the 
colour  of  a  chance  layer  on  the  surface  for  the  colour  of 
the  substance  itself.  We  see  that  best  if  we  break  it 
into  pieces,  and  so  expose  the  inner  part.    Let  us  try  it. 


PROPERTIES.  9 

Look  what  I  have  here.      It  is  a  blue  substance,  which 
is  called  copper  sulphate. 

P.  Oh,  please  don't  break  it  up,  it  has  such  a  lovely 
shape,  just  Hke  a  cut  jewel. 

M.  Those  shapes  are  called  crystals;  they  are  not 
made  by  cutting,  but  form  themselves  without  our  help. 

P.  May  I  see  that? 

M,  You  will  soon  learn  for  yourself  how  to  form 
crystals.  I  have  a  great  many  more,  and  we  can  quite 
well  use  this  one,  if  we  are  going  to  learn  anything  by  it. 
There,  I  have  broken  it:  look  closely  if  the  blue  coloui 
of  this  stuff  is  its  own. 

P.  Yes,  it  is,  because  the  stuff  is  just  as  bright  a  blue 
inside  as  outside. 

M.  Now  we  will  break  it  up  still  smaller  in  this  thick 
little  porcelain  dish,  which  is 
called  a  mortar.     For  that  we 
will  use  this  thick  rod,  which 
is  called  a  pestle  (Fig.  i). 

P.  Why  are  you  giving  your- 
self so  much  unnecessary 
bother?      We    know    already  ^^^-  ^• 

what  will  happen. 

M.  Look  at  it  carefully.  When  you  have  drawn  a  con- 
clusion you  must  test  it  properly,  or  else  you  won't  know 
that  you  haven't  overlooked  or  forgotten  something. 
What  do  you  see  ? 

P.  The  pieces  don't  seem  to  be  quite  so  blue  inside  as 
the  crystal  was  outside,  for  the  broken  bits  seem  to  get 
lighter  coloured,  and  now  the  powder  is  quite  pale  blue, 
almost  white.  I  can't  understand  that,  because  before, 
the  big  bits  looked  quite  dark  blue.  Perhaps  something 
has  been  rubbed  off  from  the  mortar? 


I  o  CON  VERSA  TIONS  ON  CHE  MIS  TR  Y. 

M.  No,  porcelain  is  hard,  and  is  not  affected  by 
rubbing.  But  look  at  these  broken  bits  of  blue  glass. 
Here  it  is  even  darker  than  the  copper  sulphate  was,  and 
here  it  is  almost  colourless,  yet  it  is  the  same  blue  glass. 

P.  That  is  quite  easy  to  explain :  the  glass  is  far  thicker 
in  one  part  than  in  the  other.  Ah,  now  I  understand; 
the  little  pieces  of  copper  sulphate  are  just  as  light  blue 
as  the  glass  in  their  thin  parts,  and  the  large  pieces  dark 
like  the  thick  glass. 

M.  Right.  When  light  penetrates  a  piece  of  the  blue 
substance  it  gets  reflected  again  and  again  inside,  till  it 
can  come  out  somewhere,  so  that  the  further  in  it  has  to 
penetrate  the  bluer  it  becomes.  That  is  why  the  larger 
or  thicker  pieces  are  darker  than  the  smaller.  In  the 
same  way  the  main  mass  of  the  sea  is  dark  blue  or  green, 
and  the  small  quantities  of  broken-up  water  seen  on 
the  foam  of  the  waves  or  in  the  track  of  a  ship  look  quite 
white.  That  is  why,  when  you  are  talking  of  the  colour 
of  a  substance,  you  must  mention  at  once  whether  you 
are  thinking  of  it  in  a  state  of  powder  or  in  big  lumps. 
Generally,  when  we  give  the  colour  of  a  thing  in  chem- 
istry, we  describe  its  colour  as  seen  when  it  has  been  arti- 
ficially prepared.  A  great  deal  still  remains  to  be  said 
on  the  question  of  colour,  but  we  have  had  enough  for 
to-day. 

3.  SUBSTANCES  AND  MIXTURES. 

M.  Go  over  what  you  learned  yesterday. 

P.  Substances  are  known  by  their  properties.  One  of 
these  properties  is  colour.  This  looks  different,  how- 
ever, according  as  the  substance  is  in  large  or  small 
pieces. 


SUBSTANCES  AND    MIXTURES.  II 

M.  Right.  Do  you  know  this  stone?  It  is  called 
granite.     What  is  its  colour? 

P.  Grey,  and  reddish,  and  black. 

M.  Why  do  you  name  several  colours? 

P.  There  are  several  in  the  stone;  there  are  grey,  and 
red,  and  black  bits.  You  can't  say  that  it  has  any  one 
colour. 

M.  Is  granite  a  substance  ? 

P.  Of  course;  because  all  sorts  of  things  are  made  of 
granite,  for  example,  the  street  pavements.  And  a  small 
piece  of  granite  is  still  granite. 

M.  Let  us  see.  Now,  just  imagine  granite  crushed 
into  such  small  pieces  that  every  separate  piece  is  either 
black,  red,  or  grey.  Then  we  put  all  the  grey  pieces  in 
a  heap  together,  and  the  same  with  the  red  and  the  black. 
Would  you  call  each  of  the  three  heaps  granite,  or  only 
one,  and  which  ? 

P.  Perhaps  the  red.  No,  that  wouldn't  do.  Granite 
is  only  granite  when  it  is  all  together. 

M.  Quite  right.  Could  you  do  the  same  with  a  piece 
of  sugar,  and  how  many  heaps  would  you  have  then? 

P.  No,  it  wouldn't  work  with  sugar.  Sugar  always 
remains  the  same. 

M.  Right  again.  Now,  notice  well,  you  have  discov- 
ered a  very  important  difference.  Substances  like  gran- 
ite, which  can  be  divided  up  into  different  heaps  after 
they  have  been  broken  up,  are  called  mixtures.  Those 
where  it  is  not  possible,  as  with  sugar,  are  of  the  same 
kind  through  and  through;  we  call  them  homogeneous. 
In  chemistry  we  only  concern  ourselves  with  homogene- 
ous substances. 

P.  Why  with  these  only  ? 

M,  Because  the  number  of  the  others  is  endless.     Just 


1«  CONVERSATIONS  ON  CHEMISTRY, 

think:  You  have  two  different  homogeneous  substances. 
Then  you  can  make  innumerable  mixtures  according  to 
the  proportions  in  which  you  mix  them.  If  we  had  to 
take  note  of  every  single  mixture,  we  should  never  come 
to  an  end. 

P.  But  after  all  they  are  something;  we  can't  leave 
them  out. 

M.  Very  good.  You  are  quite  right.  But  we  don't 
need  to  know  each  mixture  separately,  and  this  is  true 
for  the  following  reasons:  When  we  bring  together  two 
homogeneous  substances  into  a  mixture,  all  the  prop- 
erties of  the  mixture  are  such  as  can  be  calculated  from 
the  properties  of  the  two  separate  substances,  according 
to  the  proportion  in  which  they  are  mixed.  For  in- 
stance: A  mixed  colour  is  the  result  of  the  simultaneous 
and  separate  action  of  the  single  colours  of  which  it  is 
made;  the  mixing  of  colours  in  painting  depends  on  this. 
For  this  reason  we  needn't  examine  very  closely  into  the 
properties  of  mixtures. 

P.  Please  explain  that  more  clearly. 

M.  When  a  shopman  has  marked  a  yard  of  material 
at  a  certain  price,  he  doesn't  need  to  write  down  how 
much  a  half-  or  a  quarter-yard  costs ;  and  so  you  can  easily 
find  out  the  properties  of  the  mixtures  from  those  of  the 
ingredients;  and  you  don't  need  to  look  out  and  write 
down  those  of  all  possible  mixtures.  Everything  that 
can  be  asked  about  a  mixture  can  be  answered  by  cal- 
culation if  you  know  its  ingredients  and  their  relative 
amounts.  Our  silver  money,  for  example,  consists  of 
^Ya;  of  silver  and  ^/^^  of  copper,  and  the  value  of  a  pound 
of  this  metal  is  made  up  of  ^Ygy  of  the  value  of  a  pound 
of  silver  and  ^/^^  of  the  value  of  a  pound  of  copper. 

P.  I  see  that.     But  I  can't  always  tell  if  it  is  really  a 


SUBSTANCES  AND  MIXTURES,  13 

mixture.     When  I  take  my  paint-box  and  mix  blue  and 
yellow,  green  appears,  not  a  mixture  of  blue  and  yellow. 

M.  That  is  only  because  the  grains  of  colour  are  too 
small  for  you  to  recognize  singly  when  they  are  near  each 
other.  If  you  looked  at  the  mixture  through  a  micro- 
scope, you  would  sec  the  blue  grains  beside  and  on 
top  of  the  yellow  ones.  Blue  and  yellow  glass  laid  over 
each  other  make  green.  Therefore  when  the  light  from 
a  yellow  grain  goes  through  blue,  or  vice  versa,  it  be- 
comes green. 

P.  But  supposing  both  stuffs  were  white,  then  I 
couldn't  recognize  them  together  even  under  a  micro- 
scope, and  I  couldn't  tell  whether  it  was  a  mixture  or  not. 

M.  If  I  took  a  mixed  spoonful  of  sugar  and  white  sand, 
then  I  certainly  couldn't  see  that  there  were  two  things 
in  the  mixture.  But  when  I  pour  sugar  into  water,  what 
happens  then? 

P.  It  dissolves,  and  later  the  water  becomes  quite 
clear  again,  and  tastes  sweet. 

M.  And  what  happens  to  sand? 

P.  It  makes  the  water  cloudy. 

M.  And  doesn't  make  it  sweet.  Now,  if  I  pour  my 
mixture  of  sugar  and  sand  into  water,  the  water  will 
become  cloudy,  and  sweet  like  sugar.  So  I  can  tell 
them  both  together. 

P.  Yes,  it  is  so. 

M.  Why  is  it  so?  Now,  I  will  tell  you.  Colours  are 
not  the  only  properties  which  substances  possess,  and 
by  which  they  can  be  recognized  and  distinguished. 
The  behaviour  with  water  is  a  special  property,  and  this 
is  different  with  sugar  and  sand,  even  though  the  colours 
are  the  same.  Therefore  when  you  want  to  distinguish 
between  a  great  many  different  substances,  you  must 


1 4  CON  VERS  A  TIONS  ON  CHE  MIS  TR  Y. 

know  not  only  one  or  two,  but  a  great  many  of  their 
properties,  so  as  always  to  find  out  a  difference  even 
though  other  properties  seem  the  same.  That  is  why 
so  many  different  properties  of  substances  are  examined 
and  described  in  chemistry. 

Now  for  another  question.  Looking  at  the  ingredients  of 
granite,  we  might  think  that  we  could  separate  them  by 
their  colour,  so  that  we  had  them  in  different  portions. 
Do  you  think  that  you  could  in  any  way  separate  the 
mixture  of  sand  and  sugar? 

P.  It  ought  to  be  possible,  but  I  don't  know  how. 

M.  Just  look  at  the  glass  in  which  I  have  stirred  up 
the  mixture  with  water.  Now,  the  sand  has  sunk  to  the 
bottom,  and  the  sugar  is  dissolved  in  the  water. 

P.  Yes,  now  I  see;  you  only  need  to  pour  off  the  water 
with  the  sugar,  and  the  sand  will  be  left  behind  in  the 
glass. 

M.  Will  they  both  be  completely  separated  then? 

P.  No,  you  can't  pour  out  all  the  water.  The  sand 
will  be  wet,  and  some  sugar  will  still  be  in  the  water. 

M.  Now,  g,ttend  and  see  how  it  is  possible  to  do  it. 
I  have  here  a  round  piece  of  a  particular  sort  of  paper, 
which  is  called  filter-paper.  It  is  something  like  blot- 
ting-paper, as  it  sucks  up  water,  only  it  is  made  of  a 
purer  and  firmer  substance.  I  fold  the  paper  in  half, 
and  then  again  in  half,  and  pull  it  apart  so  that  a  sort  of 
little  trumpet  is  made,  which  is  quite  plain  on  one  side, 
and  on  the  other  has  three  layers  of  paper.  That  is 
called  a  filter.  I  put  my  filter  in  a  glass  funnel  and  wet  it 
with  water.  Now  I  can  press  the  filter  on  to  the  sides  of 
the  funnel  so  that  it  quite  covers  it.  The  funnel  is  now 
put  in  a  stand  and  a  glass  placed  under  it  (Fig,  2). 

P.  What  is  the  good  of  all  this? 


SUBSTANCES  AND  MIXTURES.  15 

M.  To  separate  the  sand  entirely  from  the  sugar.     If  I 
pour  the  mixture  of  sand  and  sugary  water  into  the  filter, 


Fig.  2. 


the  water  will  come  through  and  the  sand  remain  in  the 
filter. 

P.  But  the  sand  is  still  wet  and  some  sugar  is  still 
there. 

M.  That  we  will  soon  wash  out.  I  only  need  to  pour 
some  pure  water  into  the  filter,  and  this  will  run  through 
and  take  the  sugary  water  with  it.  Also,  to  rinse  the  last 
grains  of  sand  that  remain  in  the  glass  into  the  filter,  I 
use  fresh  water.  In  case  it  wasn't  completely  through 
the  first  time,  I  wait  till  the  water  has  run  through,  and 
repeat  the  rinsing  out  several  times.  So  now  we  are 
ready.  When  the  filter  with  the  sand  is  quite  dry,  then 
we  have  completely  separated  it  from  the  sugar. 


1 6  CONVERSATIONS  ON  CHEMISTRY. 

P.  But  how  are  we  going  to  get  the  sugar? 

M.  We  shall  get  that  to-morrow.  I  pour  the  water 
that  has  run  through  the  filter  into  a  flat  china  dish,  or 
a  plate,  and  place  it  on  the  warm  stove. 

P.  Why? 

M.  What  does  water  do  when  you  put  it  on  a  warm 
stove  ? 

P.  It  dries  up. 

M.  Yes,  it  evaporates,  it  changes  into  water  vapour, 
which  disappears  in  the  air,  and  nothing  is  left  in  the 
dish.  Does  sugar  do  that,  too?  Does  it  become  less 
when  it  is  on  a  warm  stove? 

P.  No,  it  stays  there  till  some  one  eats  it  up. 

M.  Quite  right.  If  I  put  my  water  which  contains 
the  sugar  in  a  warm  place,  the  water  will  evaporate,  but 
the  sugar  will  stay  behind,  and  when  all  the  water  is 
evaporated,  only  the  sugar  remains  in  the  dish.  In  this 
way  we  shall  at  last  have  completely  separated  our  mix- 
ture of  sugar  and  sand. 

P.  I  wonder  what  the  sugar  will  look  like  to-morrow. 
At  present  you  can't  see  it  a  bit,  for  the  water  is  quite 
clear,  and  to-morrow  it  ought  to  be  there  still. 


4.  SOLUTIONS. 

P.  Is  the  sugar  there  ? 

M.  Here  is  the  dish.     Look  at  it. 

P.  Yes,  I  can  see  a  white  heap  that  looks  like  sugar. 
There  is  still  some  wet,  though. 

M.  That  is  the  rest  of  the  water  which  remains  with 
the  sugar,  and  only  goes  away  slowly.  A  great  deal  of 
sugar  is  dissolved  there,  and  the  fluid  is  much  less  mobile 


SOLUTIONS. 


17 


than  pure  water,  and  the  water  takes  far  longer  to  evap- 
orate. 

P.  But  it  hasn't  come  out  in  powder  as  we  put  it  in. 

M.  No,  it  has  appeared  in  the  form  of  crystals.  The 
crystals  in  the  dish  are  not  large,  neither  distinct  nor  beau- 
tiful. But  I  have  another  sort  of  sugar  here ;  *  do  you 
know  it  ? 

P.  Yes,  it  is  sugar  candy. 

M.  Quite  right;  this  kind  of  sugar  candy  is  generally 
made  this  way.  You  dissolve  it  in  warm  water  and  let 
it  slowly  separate  out  or  crystaUize.  Only  look  care- 
fully at  the  sugar  candy;   every  piece  is  a  crystal. 

P.  Yes,  now  I  recognize  everywhere  the  smooth,  even 
sides.     Is  ordinary  sugar  not  made  of  crystals  ? 


^ 


^ 


// 


U-^ 


// 


Fig.  3. 

M.  Certainly,  only  the  crystals  are  far  smaller.  Here 
is  a  magnifying-glass,  a  lens.  Just  look  through  it  at  the 
sugar  in  the  sugar-basin. 

P.  It  looks  like  sugar  candy. 


i8 


CONyERSATIONS  ON  CHEMISTRY. 


M,  Loaf  sugar  also  consists  of  crystals,  but  they  are 
so  grown  together  that  you  cannot  easily  recognize  them. 
All  this  sugar  is  separated  from  solutions,  and  therefore 
it  is  always  crystalline;  that  means  it  is  made  of  more 
or  less  distinctly  developed  crystals. 

P.  Are  crystals  always  left  when  you  let  a  solution 
evaporate  ? 

M.  In  most  cases.  But  to  get  crystals  you  needn't 
always  let  a  solution  evaporate;  there  are  many  other 
ways,  one  of  which  I  will  show  you  immediately. 
Here  I  have  a  glass  with  the  copper  sulphate  we  used 
lately.  If  I  put  some  with  water  and  shake  it,  it  will 
dissolve,  and  the  water  will  become  blue  (Fig.  3). 
P.  Why  do  you  do  that  in  this  little  glass  tube  ? 
M.  You  will  soon  see  why.  A  chemist  uses  these 
little  tubes  for  most  of  his  experiments,  as  long  as  he  is 

not  working  with  great  quanti- 
ties, and  for  that  reason  they  are 
called  test-tubes.  Now  I  light 
my  spirit  lamp  and  heat  the 
water  with  the  copper  sulphate 
(Fig.  4)- 

P.  Take  care,  the  glass  will 
crack!  How  extraordinary!  it 
hasn't  broken. 
M.  This  sort  of  glass  doesn't  break  if  you  handle 
it  properly.  Now  look  at  the  contents;  before  there 
was  copper  sulphate  with  the  blue  water;  now  it  has 
vanished  and  the  solution  is  a  darker  blue.  I  can  put 
more  copper  sulphate  in  now,  and  it  will  also  dissolve. 
But  if  I  add  more  and  more,  finally  I  can  bring  the  solu- 
tion to  the  boil,  and  the  remainder  stays  in  the  same 
condition.      Now  I  add  some  more  water  to  it  and  heat 


Fig. 


SOLUTIONS.  19 

it  up  again,  and  it  all  dissolves.  We  will  now  put  the 
clear  liquid  aside. 

P.  But  why  didn't  the  test-tube  break  before?  Glass 
cracks  when  you  heat  it. 

M.  Not  always.  You  know  that  you  make  glass  by 
melting  it,  and  to  do  that  it  must  be  very  hot;  every  ves- 
sel or  piece  of  glass  has  been  made  very  hot,  and  yet  has 
not  cracked. 

P.  Yes,  but  mother  scolded  me  the  other  day  because 
I  had  poured  hot  wate»  into  a  glass,  and  it  had  broken. 

M.  That  is  quite  true.  Here  is  a  contradiction  which 
we  must  try  to  unravel.  In  what  other  ways  can  you 
crack  glass  ? 

P.  By  hitting  or  crushing  it. 

M.  Yes,  when  you  want  to  try  to  make  the  glass  a 
different  shape  and  at  the  same  time  try  to  strain  differ- 
ent parts  differently.  Can  heat  also  have  an  effect  on 
the  form  of  glass  ? 

P.  Yes,  heat  causes  all  bodies  to  expand. 

M.  Quite  right.  Then  a  hot  glass  will  be  rather  larger 
than  a  cold  one.     Have  you  ever  seen  that  ? 

P.  No;  it  is  so  little  that  you  can't  see  it. 

M.  All  the  same  I  will  show  you.  I  have  here  a  fairly 
long  glass  tube.  I  fasten  it  with  one  end  in  a  stand,  so 
that  it  is  horizontal,  and  put  at  the  free  end  a  measured 
ruler.  Now  notice  the  line  where  the  end  is  pointing.  So 
that  you  may  see  it  better,  I  shall  stick  on  a  black  needle 
with  wax.  Now  I  bring  my  lamp  under  the  tube  so  as 
to  heat  it.     What  do  you  see? 

P.  The  end  first  rises,  and  then  goes  slowly  down 
again  (Fig.  5).     Extraordinary! 

M.  Why  are  you  so  astonished? 

P.  I  thought  the  needle  would  go   forward,  because 


20  CONVERSATIONS  ON  CHEMISTRY. 

as  the  heat  makes  the  glass  tube  expand  it  must  get 
longer. 

M.  Instead  of  that  it  becomes  crooked,  and  bends 
upward.     How,  I  will  explain  to  you. 

P.  Wait  a  moment;  I  know  it  myself.  The  lower 
part  of  the  tube  where  the  flame  hits  it  has  become  hot- 
ter than  the  upper  part,  and  so  it  has  expanded  more 
below  than  above,  and  has  become  bent. 

M.  Right;  and  afterwards  the  upper  part  got  hot  also. 


Fig.  5. 

and  bent  itself  straight  again.  Then  glass  is  slightly 
bendable.     But  if  I  bend  it  too  roughly — 

P.  It  breaks. 

M,  Now  you  can  see  why  a  glass  breaks  with  heat. 
When  you  heat  it  unequally  it  bends,  and  when  this  hap- 
pens too  quickly,  it  breaks.  But  if  the  glass  is  equally 
warmed  this  doesn't  happen.  The  hot  water  warmed 
your  glass  in  the  inside  while  it  was  quite  cold  outside, 
and  that  is  why  it  cracked. 

P.  But  your  tube  was  cold  inside  when  you  put  it  in 
the  flame  and  heated  the  outside  of  it.  Why  didn't  it 
crack  too? 

M.  Because  it  is  made  of  very  thin  glass.  The  heat 
passed  quickly  through  the  whole  glass.     You  can  also 


SOLUTIONS.  21 

bend  thin  glass  far  more  than  thick  before  it  cracks. 
That  is  why  all  chemical  glass  apparatus  needed  for 
heating  purposes  is  made  of  thin  glass,  and  care  is  taken 
that  it  is  not  too  quickly  or  unevenly  heated,  so  that  the 
warmth  may  spread  itself  equally  over  the  whole  glass. 
But  now  we  will  look  how  our  copper  sulphate  solution 
is  getting  on,  that  in  the  mean  time  has  become  cold. 

P.  There  is  solid  copper  sulphate  again  in  the  glass. 

M.  I  will  pour  the  liquid  part  in  another  glass,  and 
take  out  the  hard  part  with  a  glass  rod.  To  dry  it,  I  lay 
it  on  a  piece  of  filter-paper,  that  will  suck  up  the  liquid. 
Watch  it  carefully.     What  do  you  see? 

P.  There  are  crystals  again. 

M.  Yes;  these  crystals  have  not  come  because  the 
solution  is  dried  up,  but  because  it  has  cooled. 

P    Please  explain  that  to  me. 

M.  If  you  take  a  certain  amount  of  water  and  dissolve 
copper  sulphate  in  it,  can  you  dissolve  as  much  copper 
sulphate  as  you  wish? 

P.  No;  after  a  time  it  won't  dissolve  any  more. 

M.  Right.  A  given  amount  of  water  can  dissolve  only 
a  given  amount  of  another  substance.  Such  a  solution  is 
called  ''saturated."  If,  however,  you  warm  such  a  solu- 
tion, then  it  can  dissolve  more.  But  when  you  cool  it 
again,  the  solution  cannot  contain  the  extra  amount  it 
has  taken,  and  this  separates  itself  in  a  solid  form  and 
takes  the  shape  of  crystals. 

P.  That  is  just  the  same  as  after  evaporation;  for 
the  water  went  away,  and  there  was  no  more  there  for 
the  substance  to  keep  in  solution. 

M.  Quite  so.  Whenever  there  is  more  substance 
than  a  saturated  solution  can  hold,  it  separates  itself  in 
solid  form.     Later  on  we  will  learn  another  condition 


22  CONyERSATlOm  ON  CHEMISTRY. 

that  must  be  fulfilled  by  this.  But  I  haven't  yet  asked 
you  what  you  learned  yesterday. 

P.  Yesterday  we  talked  about  mixtures  and  homo- 
geneous substances.  Mixtures  consist  of  different  sub- 
stances. 

M.  And  how  can  mixtures  be  recognized  and  sepa- 
rated ? 

P.  By  the  constituents  having  different  properties;  for 
example,  we  can  pick  them  out  if  they  have  different 
colours,  or  one  will  dissolve  in  water  and  the  other  remain 
behind. 

M.  Yes,  if  the  other  doesn't  also  dissolve  in  water. 
But  the  solutions  that  are  produced,  are  they  mixtures  or 
homogeneous  substances? 

P.  Mixtures. 

M.  Why? 

P.  Because  you  can  put  them  together  out  of  different 
substances,  and  again  you  can  divide  them  up  into  their 
ingredients. 

M.  That  is  right  so  far;  but  have  solutions  like  other 
mixtures  the  same  properties  as  the  ingredients  before 
they  are  mixed? 

P.  Yes,  the  solution  of  copper  sulphate  is  blue  like  the 
copper  sulphate  and  a  solution  of  sugar  tastes  sweet  Hke 
sugar. 

M.  Copper  sulphate  and  sugar  are  soHd  bodies,  but  their 
solutions  are  liquid  like  water.  If  you  take  another  solid 
body  like  sand,  and  stir  it  up  with  water,  it  will  make  a 
thick  mixture,  and  not  a  solution. 

P.  Yes,  there  is  a  difference  there.  But  perhaps  the 
sugar  gets  divided  up  into  such  small  pieces  that  they 
can  neither  be  seen  nor  felt. 

M,  You  may  believe  it,  but  you  cannot  prove  it.    For 


MELTING  /iND  FREEZING,  23 

when  you  look  at  a  solution  even  through  the  strongest 
microscope,  you  don't  see  any  separate  particles. 

P.  But  perhaps  the  particles  are  still  smaller? 

M.  It  is  useless  to  speak  about  it  any  longer,  as  we 
can't  decide  it. 

P.  There  is  something  special  about  solutions,  then, 
which  can  be  distinguished  from  ordinary  mixtures? 

M,  Yes;  solutions  are  homogeneous  mixtures. 


5.  MELTING  AND  FREEZING. 

M.  What  did  we  speak  about  yesterday? 

P.  About  solutions,  but  I  didn't  quite  grasp  it  aU. 

M.  What  is  the  difficulty? 

P.  That  out  of  a  solid  substance  or  a  liquid  a  real 
liquid  is  made. 

M.  Just  think  for  a  minute  if  you  can't  make  liquids 
out  of  solid  substances  in  any  other  way. 

P.  Oh  yes,  when  ice  or  snow  melts. 

M.  Does  that  only  happen  with  ice  or  snow,  or  can 
other  soUd  substances  melt  ? 

P.  Yes;  on  New  Year's  eve  we  melted  lead. 

M.  Through  warming  or  heating  you  can  make  solid 
things  melt,  or  turn  them  into  liquid.  And  when  the  liquid 
is  cooled? 

P.  It  becomes  soHd  again. 

M.  Then  we  can  change  ice  into  water,  and  water 
into  ice,  if  we  warm  the  ice,  or  cool  the  water.  At  what 
temperature  will  ice  become  liquid? 

P.  At  0°. 

M.  And  when  does  water  freeze  to  ice  ? 

P.  Again  at  0°. 


2i 


24  CONVERSATIONS  ON  CHEMISTRY. 

M.  Does  the  ice  become  liquid  when  it  is  warmed  to  0°  ? 
P.  It  ought  to. 

M.  You  have  forgotten  what  you  learned  about  that 
in  your  Physics  lessons.      We  will  just  try  it  for  our- 
^  ^     selves.     I  have  here  a  thermometer.     This   sort 
I,  ^^  \      is   made  out    of    a   narrow   glass    tube,    with  a 
bulb  at   the    bottom  containing    mercury    (Fig. 
6).        As  mercury    expands    with     heat     much 
quicker     than     glass,    it    rises    higher    in     the 
tube,    and    the    higher    the  temperature    is,   the 
higher  it   rises.     A  row  of  equidistant    strokes, 
with   numbers,   a    scale,  makes    it     possible    to 
read   the   height  of    the    mercury,    and    conse- 
quently the  temperature.     I  now  dip  the  buib 
of  the  thermometer  into  the  crushed  ice  here  in 
the  beaker.    In  a  short  time  it  sets  itself  opposite 
the  stroke  with  the  mark  0°. 

P.  Why  does  the  mercury  stand  at  the  0°  ? 
M.  The    thermometer-maker   arranged    that. 
When  he  had  the  instrument  so  far  ready  that 
only  the  scale  remained  to  be  put  on,  he  put  it 
in  melting  ice,  and  marked  the  place  where  the 
\  /     mercury  was.    After  that  he  placed  the  scale  so 
I       that  the  zero  came  exactly  on  this  place, 
i  P.  Then  there  is  no  heat  there. 

^°*  *  M.  No,  it  is  a  temperature  that  we  have  called 
0°.  It  is  quite  an  arbitrary  choice,  because  you  know 
that  in  winter  the  temperature  falls  far  below  zero. 
The  lowest  temperature  that  has  been  reached  sc  far 
lies  about  260°  below  0°. 

P.  Why  did  they  hit  upon  this  choice? 
M.  That  you  will  soon  see.     I  surround  the  beaker  with 
my  hands,  and  try  to  warm  it.     Look  at  the  thermometer. 


2i;o 


MELTING  AND  FREEZING,  25 

P.  It  is  Still  at  o^. 

M.  Now  I  pour  some  water  out  of  the  bottle  that  has 
stood  in  the  room  for  this  purpose.  About  what  tempera- 
ture is  this  water? 

P.  In  a  room  it  should  always  be  about  17°  or  18°. 
The  water  will  be  about  the  same. 

M.  Look  at  the  thermometer. 

P.  It  is  at  5°. 

M.  The  warm  water  has  raised  the  temperature  then. 
Now  stir  it  carefully  round. 

P.  Now  the  thermometer  is  getting  lower;  now  it  is 
again  at  0°  and  is  remaining  there.  How  is  that?  The 
room  is  warmer,  and  the  thermometer  ought  tt)  rise. 

M.  When  ice  and  water  are  together,  the  temperature 
always  remains  at  0°  as  long  as  both  are  present.  If  you 
try  to  raise  the  temperature  by  adding  heat,  so  much  ice 
melts  as  to  use  up  the  whole  added  heat.  If  you  take 
heat  away,  so  much  water  freezes  as  to  replace  the  heat 
removed. 

P.  Is  heat  made  when  water  freezes? 

M.  Certainly;  when  water  freezes  to  ice,  exactly  the 
same  amount  of  heat  is  formed  as  is  used  when  the  ice  is 
melted  again. 

P.  How  is  it  that  it  is  exactly  the  same? 

M.  Just  suppose  for  a  moment  that  the  two  quantities 
were  different;  suppose  that  on  freezing,  the  resulting 
heat  was  represented  by  the  number  80,  and  on  melting 
only  60  was  used.  If  we  freeze  water,  and  then  let  the 
ice  melt,  it  is  exactly  the  same  at  the  end  as  at  the  begin- 
ning; but  of  the  heat,  80  parts  have  been  produced,  and 
only  60  used,  so  that  20  remain  over.  Now  this  can  be 
done  as  often  as  you  Hke,  so  that  you  could  produce  any 
quantity  of  heat  from  nothing.     But  that  is  not  possible , 


26  CONyERS^TIONS  ON  CHEMISTRY. 

and  therefore  in  melting,  exactly  as  much  heat  is  used  as 
was  given  out  on  freezing. 

P.  Is  it  quite  impossible  to  make  heat  out  of  nothing  ? 
Rubbing  makes  heat. 

M.  But  not  for  nothing.  To  rub,  you  must  work, 
and  you  cannot  create  work  out  of  nothing.  But  let  us 
leave  this  subject,  for  I  will  explain  to  you  later  what  a 
quantity  of  heat  is,  and  how  it  is  measured.  We  will  go 
back  to  our  water  and  ice.  You  saw  that  when  both  were 
together,  the  thermometer  always  remained  at  a  particular 
temperature,  which  is  generally  called  o°.  Therefore  there 
is  quite  a  definite  temperature  when  solid  ice  changes  into 
liquid  water,  or  melts.  Now,  do  you  think  that  there  is 
always  a  particular  temperature  when  a  solid  substance 
melts  ? 

P.  There  must  be  something  of  the  sort,  as  lead  is  easily 
melted,  and  silver  is  difficult  to  melt. 

M.  Now  we  come  to  a  general  law,  that  every  substance 
melts  at  a  particular  temperature  and  freezes  at  the  same 
temperature.  The  melting-point  and  freezing-point  of  a 
substance  are  always  the  same.  It  is  that  temperature 
at  which  the  solid  substance  and  the  liquid  substance  can 
exist  together,  and  at  which  heat  added  or  removed  is 
used  only  in  changing  the  liquid  or  the  solid  from  one  into 
,the  other.  The  melting-point  then  is  as  much  a  property 
as  its  colour  or  solubility. 

P.  Who  made  this  law? 

M.  The  name  law  is  only  used  figuratively.  People 
found  that  it  was  the  case  with  substances,  and  have 
consequently  compared  them  with  obedient  pupils,  who 
always  do  what  they  are  told.  In  science,  people  under- 
stand by  a  law  something  that  applies  to  many  things, 
and  can  be  expressed  in  a  general  form. 


MELTING    ^ND  FREEZING.  27 

P.  Are  there  many  laws  li'ie  that? 

M.  Yes,  a  great  many.  To  know  such  laws  makes  the 
task  of  noticing  and  using  individual  facts  much  easier. 

P.  Please  explain  that  more  distinctly. 

M.  Let  us  take  the  law  that  a  mixture  of  water  and  ice 
has  always  a  dofmite  temperature.  If  a  thermometer- 
maker  in  London  has  made  his  thermometer  so  that  a 
mixture  of  ice  and  water  shows  a  temperature  of  0°, 
he  can  be  perfectly  sure  that  wherever  in  the  whole  world 
ice  and  water  are  brought  together  the  temperature 
will  be  0°.  Were  this  not  the  case  he  couldn't  sell 
a  thermometer,  and  we  couldn't  use  a  bought  one  for  our 
purpose. 

P.  It  is  really  nice  of  the  law  to  help  the  thermometer- 
maker  so  much. 

M.  A  law  of  nature  is  not  a  being  who  either  does  some- 
thing, or  leaves  it  undone.  People  have  discovered  that 
ice  and  water  together  have  always  the  same  temperature. 
Therefore,  in  this  case,  the  thermometer-maker  is  placed 
in  such  a  position  that  he  can  always  make  generally 
useful  thermometers.  But  with  one  point,  the  point  of 
zero,  the  thermometer  is  not  finished;  all  the  other  lines 
have  to  be  marked. 

P.  Aren't  these  just  ordinary  millimetres,  like  a  ruler? 

M.  No,  that  wouldn't  work.  For  sometimes  the  tube  is 
narrow  and  sometimes  wider,  sometimes  the  bulb  with 
the  mercury  is  large,  sometimes  smaller.  The  mercury 
would  then  rise  to  different  heights  if  the  thermometers 
were  equally  warmed,  and  so  they  wouldn't  agree. 

P.  That  is  true.  Then  you  must  warm  all  thermom- 
eters the  same  amount,  and  mark  the  place  of  the  mercury, 
and  then  put  on  equal  numbers  of  marks  till  you  come  to  0°. 

M.  Good.   To  what  temperature  should  you  heat  them  ? 


28  CONVERSATIONS  ON  CHEMISTRY. 

P.  To  any. 

M.  That  wouldn't  be  right.  Of  course  all  thermome- 
ters would  agree  that  had  been  made  at  the  same  time, 
but  at  another  place  no  one  would  know  what  the  com- 
mon temperature  was  where  the  top  mark  was  made. 

P.  Then  I  can't  think  of  anytl>ing  better. 

M.  It  would  help  us  if  we  could  only  find  a  tempera- 
ture that  was  as  easy  and  certain  as  the  ice-point. 

P.  Ah,  now  I  remember;  it  is  the  boiling-point  of  water. 

M.  Yes,  it  is  the  temperature  at  which  water  boils. 
That  is  what  we  shall  speak  about  to-morrow. 


6.  BOILING  AND  EVAPORATION. 

M.  What  did  you  learn  yesterday? 

P,  I  learned  that  melting  ice  always  shows  the  same 
temperature,  which  never  alters  whether  much  or  little 
water  or  ice  is  present. 

M.  And  what  about  the  freezing  of  water  ? 

P.  That  shows  the  same  temperature.     But  what  hap- 
pens when  all  the  water  is  frozen? 
•M.  Then  we  have  only  ice,  and  this  we  can  cool  as 
much  as  we  like.     In  the  same  way,  when  we  melt  ice  all 
the  ice  becomes  Hquid .... 

P.  Then  we  have  only  water,  and  this  we  can  warm 
as  much  as  we  like. 

M.  That  is  nearly  right,  but  you  jumped  to  a  too 
rapid  conclusion,  because  it  doesn't  hold  in  all  cases- 
We  will  speak  about  this  shortly.  But  first  let  us  go 
over  again  what  we  spoke  about.  What  is  the  condition 
that  gives  the  temperature  of  o°?  Try  to  explain  this  as 
quickly  and  generally  as  possible. 


BOILING  AND  EVAPORATION,  29 

P.  Let  me  think  a  minute.  Ice  is  at  0°  when  it  melts, 
and  water  when  it  freezes.  But  when  ice  is  melted,  or 
water  is  frozen,  it  isn't  at  0°  any  longer.  There  must  be  ice 
in  water,  or  water  along  with  the  ice.  Oh,  now  I  know; 
when  ice  and  water  are  together,  then  the  temperature  is 
at  0°. 

M.  Right;  that  is  the  condition.  Can  you  see  exactly 
why  this  condition  must  be  fulfilled? 

P.  It  seems  to  me  it  must  be  quite  simple,  only  I  can't 
get  it  out. 

M.  It  is  really  quite  simple.  What  happens  when  you 
try  to  warm  a  mixture  of  ice  and  water? 

P.  You  explained  that  to  me  yesterday.  It  only  melts 
some  ice,  and  that  uses  up  the  heat  that  has  been  put  in. 

M.  And  when  you  try  to  cool  it  ? 

P.  Then  some  of  the  water  freezes  to  ice,  and  gives  .  .  . 

M.  And  gives  out  exactly  the  same  amount  of  heat 
as  has  been  taken  away.  You  see  the  thing  is  like 
the  height  of  water  in  a  pond  that  always  remains  at  the 
same  level.  If  you  take  water  away,  more  flows  in  from 
the  spring;  if  you  pour  water  in,  it  runs  over  the  dam, 
and  the  height  of  the  water  is  still  the  same. 

P.  I  understood  that,  but  I  haven't  got  it  quite  clear 
yet.  Does  a  lot  of  water  with  a  little  ice  give  the  same 
temperature  as  a  lot  of  ice  with  a  little  water? 

M.  You  have  not  been  attending.  We  learned  all  this 
yesterday  as  a  law  of  nature;  that  is  to  say,  as  a  thing 
that  is  always  the  same. 

P.  Oh,  now  I  remember;  now  I  see  it  all.  Why,  it  is 
ridiculously  easy;  I  thought  it  would  be  far  more  difficult. 

M.  That  will  often  happen.  When  you  have  got  a 
thing  quite  clear,  it  always  seems  very  easy.  But  the 
YpXing  it  clear  is  not  always  so  simple  and  easy.     But 


30 


CONVERSATIOhlS  ON  CHEMISTRY, 


now  let  us  go  back  to  my  first  remark.     Can  you  really 
heat  water  without  ice  as  much  as  you  want?     What 
happens  when  I  put  water  in  a  pot  over  the  fire? 
P.  First  it  will  get  hot,  and  then  it  will  begin  to  boil. 
M.  Right.     We  will   make   the   experiment.     I   have 

here  a  flask  made  of  thin 
glass  which  I  can  put  over 
the  flame  without  its  cracking. 
In  it  is  some  water,  and  I 
shall  put  it  over  a  tripod 
which  stands  above  my  lamp 
(Fig.  7)- 

P.  Why  is  that  wire  gauze 
on  the  tripod? 

M.  For  one  thing,  so  that 
I  can  put  large  and  small 
vessels  on  it.  Again,  the 
metal  spreads  the  heat  of 
the  flame,  and  prevents  the 
glass  from  breaking  so 
easily  if  it  is  a  little  thicker. 
Now  I  put  my  thermometer 
in  the  water. 
P.  Do  you  see?  The  water  is  getting  warmer. 
M.  Wait  a  bit. 

P.  Now  the  water  is  boiling,  and  the  mercury  has 
risen  quite  high;  it  is  already  at  ioo°.  Now  it  will  soon 
fill  the  whole  thermometer.  What  will  happen  when  the 
mercury  has  no  more  room  to  expand? 

M.  The  thermometer  will  break,  for  it  exerts  very 
strong  pressure. 

P.  Then  take  the  lamp  away  at  once. 
M.  Look  at  the  thermometer  first. 


Fig.  7. 


BOILING  AND  EVAPORATION,  31 

P.  It  is  Still  at  100°. 

M.  And  will  stay  there  as  long  as  you  like.  I  am 
making  the  flame  bigger.     What  do  you  see? 

P.  The  water  is  boiling  harder. 

M.  And  the  thermometer? 

P.  That  is  still  at  100°.  Oh,  now  I  am  beginning  to 
notice  something.  It  seems  to  be  exactly  the  same  here 
as  with  the  melting. 

M.  Quite  right.  Now  try  to  trace  the  resemblance. 
Then  the  temperature  was  unchangeable  when  two 
things,  ice  and  water,  were  together.     What  is  it  here? 

P.  There  is  water  here  too,  but  what  is  the  second? 
Wait  a  bit,  I've  got  it  now;  it  is  steam.     Is  that  right? 

M.  Yes.  When  I  supply  heat  by  means  of  a  flame,  it 
doesn't  heat  the  water  any  more,  but  changes  it  .  .  . 

P.  Into  steam! 

M.  Now  we  must  reverse  this  relation.  We  had  the 
same  temperature  before,  whether  we  started  with  water 
or  with  ice;   now  .  .  . 

P.  Now  we  must  get  the  same  temperature  whether 
we  start  with  water  or  steam.  We  have  got  the  one  when 
we  started  with  water,  but  how  do  we  get  the  other? 
We  must  take  a  vessel  with  steam,  and  try  to  cool  it. 
That  isn't  easy  to  do;  we  must  have  a  boiler  for  it. 

M.  We  can  do  it  in  a  much  easier  way.  Look,  I  take 
the  thermometer  out,  and  let  the  water  boil  quickly  for  a 
minute  or  two.  The  thermometer  has  now  cooled  a  little, 
and  it  has  fallen  to  under  50°.  Now  I  put  it  again  into  the 
flask;  not  into  the  boiling  water,  however,  but  hold  it 
above  in  the  upper  part  of  the  flask.     What  do  you  see  ? 

P.  Water  is  dropping  from  the  thermometer.  How 
did  it  get  there?  I  know;  the  steam  in  the  upper  part 
of  the  flask  has  condensed  on  the  cold  thermometer. 


32  CONVERSATIONS  ON  CHEMISTRY, 

M.  Right.     Read  the  temperature. 

P.  It  is  at  1 00°  again. 

M.  Now  we  have  made  the  experiment  for  which  you 
wanted  a  boiler.  The  upper  part  of  the  flask  contains 
steam,  which  rushes  upwards  and  makes  clouds  outside. 
By  the  cold  of  the  thermometer  a  part  of  the  steam  is 
made  into  liquid  water,  and  also  in  the  upper  part  of  the 
flask  too.  You  have  thus  steam  and  water  together. 
Steam  condenses  to  water  on  the  thermometer  till  the 
lost  heat  is  supplied  again,  and  the  temperature  has  risen 
to  100°. 

P.  Is  there  really  steam  in  the  upper  part  of  the  flask? 
It  is  quite  clear. 

M.  Steam  is  as  transparent  as  air. 

P.  Is  that  so?  I  thought  steam  was  always  misty  and 
untransparent.  When  a  steam-engine  blows  out  steam, 
you  see  it  like  a  thick  white  cloud,  and  in  the  same  way 
the  clouds  in  the  sky  are  steam. 

M.  No,  what  you  see  isn't  steam,  but  liquid  water  in 
very  small  drops  that  have  been  made  out  of  steam  by  cool- 
ing. If  you  could  look  into  the  boiler  of  a  steam-engine, 
you  would  see  that  the  inside  is  quite  clear,  just  as  if  it 
was  full  of  air.  Also  in  the  clearest  air  there  is  always  a 
large  amount  of  steam;  and  mist  and  clouds  are  made  up 
by  the  cooling  and  building  up  of  liquid  water  in  the  shape 
of  tiny  drops.  So  you  see  these  things  behave  in  very 
much  the  same  way  as  water  and  ice.  Water  and  steam 
only  exist  together  at  a  definite  temperature,  and  when  they 
are  present  together  that  must  be  the  temperature. 

P.  How  does  it  happen  that  it  is  exactly  100°  ? 

M.  In  every  thermometer  the  100°  is  marked  just  where 
the  mercury  rises  to  in  boiling  water. 

P.  How  can  they  do  that? 


BOILING  AND  EyAPORATtON. 


33 


100- 


-212 


50- 


M.  Don't  you  remember  how  we  left  the  thermometer- 
maker?  He  had  only  been  able  to  put  one  mark  on  his 
tube,  and  had  written  o°  there  when  the  mercury  was  in 
melting  ice.  Now,  he  must  have  another  distinct  tem- 
perature to  have  another  mark,  in  order  that  he  may 
divide  up  his  instrument.  This  second  temperature  is 
that  of  boiHng  water,  and  people  came  to  the  conclusion 
that  the  portion  between  the  two  marks 
was  to  be  divided  into  a  hundred  parts. 
As  the  lowest  mark  is  called  o°,  the  top 
one  must  be  ioo°. 

P.  Now  I  understand.  But  how  can 
higher  or  lower  temperatures  be  measured  ? 

M.  As  many  equal  divisions  are  marked 
below  the  zero-point  and  above  the  loo- 
point  as  there  is  room  for.  According  as 
the  thermometer  is  required  for  high  or 
low  temperatures,  more  or  less  mercury 
is  put  in,  so  that  there  is  enough  space 
over  on  the  required  part  (Fig.  8). 

P.  But  our  window-thermometer  is  not 
divided  up  to  ioo°.  It  stops  at  50°. 
How  could  they  make  the  right  division 
in  that  case? 

M.  First  a  thermometer  is  made  with 
great  care  from  0°  to  100°  and  correctly 
divided.  That  is  called  a  normal  ther- 
mometer. Then  the  short  thermometer 
is  brought  into  the  same  medium  as  this 
— for  instance,  both  are  dipped  into  a  rather  large 
quantity  of  water.  Since  obviously  both  thermometers 
must  now  register  the  same  temperature,  we  have  merely 
to  mark  on  the  small  one  the  number  at  which  the  mer- 
cury stands  in  the  large  one. 


— 100 


Fig.  8. 


34  CON  VERS  A  TIONS  ON  CHE  MIS  TR  Y. 

P.  Is  that  how  it's  done?  Now,  I  don't  think  I've 
anything  more  to  ask.  Yes,  I  have  though ;  on  the  left 
of  our  window- thermometer  is  a  C  and  on  the  right  an 
F.,  and  it  is  divided  differently  on  both  sides. 

M.  F.  means  Fahrenheit.  Fahrenheit  was  a  German 
who  made  the  first  comparable  thermometer;  he  lived 
in  the  eighteenth  century.  He  wanted  to  divide  his 
thermometer  from  the  lowest  temperature  that  there 
was;  so  he  put  it  in  a  mixture  of  snow  and  sal  ammoniac, 
and  marked  the  point  to  which  the  mercury  sank  as  o°. 
The  piece  between  this  point  and  freezing-point  of  water 
he  divided  into  32  parts,  and  found  that  180  of  these 
parts  were  contained  in  the  space  between  freezing-point 
and  boiling-point.  People  used  the  division  of  Fahren- 
heit for  this  reason  alons,  because  the  freezing-point  was 
32°  and  the  boiling-point  32° -f  180°,  or  212°. 

P.  Why  don't  they  still  use  Fahrenheit's  plan? 

M.  Because  the  mixture  of  sal  ammoniac  and  snow 
is  very  difficult  to  bring  to  a  definite  temperature, 
while  the  freezing-  and  boiling-points  of  water  are  much 
surer. 

P.  Does  every  one  use  these  thermometers? 

M.  The  English  and  Americans  do.  They  use  them 
only  in  ordinary  life,  however,  mostly  for  open-air  ther- 
mometers. In  all  scientific  work  they  use  the  centigrade 
thermometer.  Give  me  the  equation  between  Celsius 
and  Fahrenheit,  and  use  the  letter  /  for  Fahrenheit,  and 
c  for  Celsius. 

P.  /:c=i8o°:ioo°,  or  5/  =  9C. 

Mo  That  is  not  right. 

P.  Why  not? 

M.  The  freezing-point  of  Celsius  is  zero.  If  you  say 
c  =  o°,  then  your  equation  comes  out  to  /  =  o°.     But  the 


BOILING  AND  EVAPORATION.  35 

freezing-point  of  Fahrenheit  is  not  o°,  but  32°.     What 
must  you  do  so  as  to  make  /  =  32°  when  c  =  o°? 

P.  I  must  put  the  32  on  the  other  side. 

M.  Well,  let  me  hear  the  equation. 

M.  Put  the  c  =  o  in  here.   Now  what  happens? 

P.  5/  =  32.  No,  that  is  not  right;  the  /  must  stand 
alone  on  the  left.  How  can  I  do  that?  Now  I  know: 
First,  I  must  write  ]=^/ f,c  and  then  add  32  to  the  right; 
so  /=V5C+32.     Now,  I  put  c  =  o°  and  that  comes  right; 

M.  Yes,  now  the  equation  is  right. 

P.  I  have  read  about  a  thermometer  called  Rdaumur 
that  was  quite  different. 

M.  Yes.  For  rather  more  than  a  hundred  years  the 
thermometer  of  a  Frenchman,  Reaumur,  has  been  used. 
In  his,  the  space  *between  freezing-  and  boiling-point  was 
divided  not  into  100,  but  80  parts.  On  the  other  hand, 
the  Swede  Celsius  introduced  the  division  of  100.  In 
Germany  the  Reaumur  thermometer  came  into  use,  while 
in  France  the  centigrade  one  was  used.  Presently  people 
grew  accustomed  to  register  all  temperatures  by  the 
centigrade  thermometer;  in  science  no  other  is  now  used. 
What  is  the  relation  between  the  degrees  of  Reaumur  and 
Celsius  ? 

P.  100°  C.  are  80°  R. 

M.  Simplify  the  proportion. 

P.  10°  C.  are  8°  R.,  or  5°  C.  are  4°  R. 

M.  You  can  write  this  as  an  equation  too.  Take  c  for 
centigrade  and  r  for  Reaumur  degrees — that  makes 
c:r::5:4,  so  c=^/^r^  or  r^*/^c.    The  first  equation  you 


36  CONVERSATIONS  ON  CHEMISTRY. 

use  when  you  wish  to  change  Reaumur  into  centigrade, 
and  vice  versa. 

P.  Has  the  mixture  of  ice  and  the  other  thing  really — 

M,  Sal  ammoniac  ? 

P.  And  sal  ammoniac  the  lowest  temperature  that  there 
is? 

M.  Far  from  itl  It  is  sometimes  colder  here  in  winter. 
Think  how  many  degrees  of  Celsius  there  are  to  the 
zero  of  Fahrenheit. 

P.  I   must  put   /  =  o;    then  0  =  ^/^0+32;  that    makes 

M.  Yes,  not  quite  18°  under  0°;  but  in  America  it  is 
often  20°  to  25°  below  zero. 

P.  What  is  the  greatest  cold  that  there  is? 

M.  Up  to  the  present  259°  below  0°  has  been  attained. 

P.  What  do  you  mean?^   Will  they  get  further? 

M.  Not  much.  Probably  —  273°C.  is  the  lowest  tem- 
perature there  is. 

P.  Why  do  you  think  that  ? 

M.  I  can't  explain  to-day,  but  you  will  soon  discover 
and  believe  it  as  well. 

P.  Oh,  I  wish  I  knew! 

7.  MEASURING. 

M.  What  did  you  learn  yesterday? 

P.  How  thermometers  are  made. 

M.  Yes.  As  a  thermometer  is  a  sort  of  measuring 
instrument  we  will  speak  a  little  about  measurement. 
What  can  be  measured? 

P.  All  sorts  of  things:  Lengths,  weights,  surfaces.  I 
think  almost  everything  can  be  measured. 


MEASURINC.  37 

M.  Not  all,  but  a  great  many  things.  What  is  used 
for  measuring? 

P.  A  measure. 

M.  What  is  that? 

P.  There  are  different  sorts;  it  depends  on  what  you 
want  to  measure. 

M    Give  me  an  example. 

P.  Well,  the  length  of  the  table  can  be  measured  in 
feet  and  inches. 

M.  Although  feet  and  inches  are  used  in  England  and 
America,  all  scientific  people  measure  in  what  is  called 
the  metric  system. 

P.  What  is  that? 

M.  We  are  going  to  learn  it.  Here  is  a  centimetre  rule. 
Measure  the  length  of  the  table. 

P.  The  scale  is  50  centimetres  long;  I  see  that  on  the 
last  figure.  I  lay  the  measure  so  that  its  end  is  against 
the  end  of  the  table,  and  notice  to  where  it  reaches. 
Then  I  put  the  measure  at  that  mark,  and  again  make  a 
scratch  where  it  ends.  My  measure  comes  beyond  the 
table,  now  that  I  have  put  it  at  the  second  mark,  and  I 
look  at  which  number  the  table  ends.  It  is  at  22.  So 
the  table  is  50+ 50-f  22^=122  cm. 

M.  Quite  right.  You  went  on  adding  centimetres 
together  till  you  had  got  the  same  amount  as  the  length 
of  the  table.  The  measure  only  helped  you  to  count 
the  centimetres. 

P.  Yes,  so  it  did. 

M.  And  how  do  you  set  about  measuring  weights? 

P.  I  put  the  thing  in  one  pan  of  a  balance,  and  add 
weights  to  the  other,  till  they  are  both  the  same 
weight. 

M,  And  how  can  you  notice,  or  tell  the  weight  ? 


38  COhiyERSATIONS  ON  CHEMISTRY. 

P,  The  number  of  ounces  each  one  weighs  is  marked 
on  the  weight;    I  add  the  figures  all  up  afterwards. 

M.  Let  us  use  grams.  You  see  it  is  the  same  as  before; 
you  add  grams  together  till  their  weight  is  the  same  as 
that  of  the  object.  The  weights  only  help  you  to  count 
the  grams. 

P.  So  they  do.     I  never  noticed  that  both  were  so  like. 

M.  You  will  soon  see  that  all  real  measuring  is  based 
upon  the  same  principle.  But  now  for  another  question: 
Why  didn't  you  measure  the  length  with  grams  and  the 
weight  with  centimetres? 

P.  It  wouldn't  work. 

M.  Why  not? 

P.  However  many  centimetres  I  put  together  they 
would  never  make  a  weight. 

M.  Quite  right.     Put  this  in  a  simple  form. 

P.  Length  can  be  only  measured  by  length,  and 
weight  by  weight. 

M.  It  could  be  said  still  more  simply.  Every  quantity 
can  be  measured  by  a  like  quantity. 

P.  Yes,  I  understand  that. 

M.  You  measured  length  in  centimetres,  hre  centi- 
metres the  only  measure  of  length? 

P.  No,  there  are  millimetres,  kilometres,  inches,  miles, 
fathoms,  and  a  great  many  others. 

M.  How  far  do  these  differ  ? 

P.  A  centimetre  has  a  different  length  from  an  inch, 
and  so  on. 

M.  Yes;  these  definite  lengths,  such  as  a  centimetre, 
inch,  and  mile,  are  called  units  of  length.  In  every  state- 
ment of  measurement  we  get  the  kind  of  unit  which  has 
been  used,  and  the  number- of  units  which  are  contained 
in  the  thing  measured. 


MEASURING. 


39 


Fig.  9. 


P.  Then  why  are  there  so  many  sorts  of  units  for  the 
same  sort  of  quantity;  for  example,  length? 

M.  That  is  because  the  choice  of  the  units  is  arbi- 
trary. At  first  different  groups  of  people  who  required 
a  unit  of  length  chose  one  without 
troubling  themselves  about  what 
other  people  were  using.  Finally 
these  differences  grew  so  un- 
bearable that  in  France,  at  the  end 
of  the  eighteenth  century,  the  State 
determined  to  abolish  the  old  meas- 
urement and  to  use  a  new  one  in 
its  place.  It  was  determined  to 
protect  the  standard  against  acci- 
dental destruction,  and  so  it  was 
decided  to  use  the  world  itself  as  a  measure.  The 
length  of  a  quadrant  of  the  meridian,  that  is,  the  length 
from  ^  to  A^  (Fig.  9),  was  divided  into  ten  million 
parts,  and  these  parts  were  called  metres,  and  were 
to  serve  as  a  common  unit  of  length.  A  centimetre  is 
a  hundredth  part  of  this  length,  so  that  it  is  a  thousand- 
millionth  of  the  earth's  quadrant. 

P.  But  how  can  the  earth's  quadrant  be  divided, 
when  no  one  has  been  to  the  north  pole  ? 

M.  Only  a  part  of  it  is  measured,  the  relation  of  which 
to  the  whole  is  determined  by  the  angle  which  lines  at 
right  angles  to  two  tangents  form  with  each  other.  But 
it  turned  out  that  this  measurement  was  far  less  accurate 
than  the  comparison  of  two  metre  scales.  Accordingly 
the  metre  was  taken  to  be  the  length  of  a  standard  kept 
in  Paris,  made  of  the  most  indestructible  material  which 
could  be  found — an  alloy  of  the  noble  metals  platinum 
and  iridium. 


40  CONyBRSATlONS  ON  CHEMISTRY. 

P.  But  supposing  this  scale  was  lost  or  got  destroyed? 

M.  Care  is  taken  about  that.  Twenty  similar  scales 
have  been  made,  all  carefully  compared  with  each  other, 
and  there  is  one  at  Berlin,  London,  New  York,  St.  Peters- 
burg, Rome,  and  many  other  places,  so  that  any  one  of 
them  might  be  lost  without  the  loss  of  the  standard. 
Then,  again,  many  other  scales  made  of  different  materials 
have  been  compared  with  them,  so  that  the  permanence 
of  the  unit  is  about  as  certain  as  that  of  the  human  race. 

P.  But  the  metre  is  quite  an  arbitrary  measure.  Why 
hasn't  one  been  chosen  which  is  free  from  man's  control? 

M.  Because  there  is  practically  none. 

P.  But  with  angles  it  is  different.  I  have  learned  in 
my  geometry  class  that  a  right  angle  is  a  natural  measure 
which  cannot  be  altered.  Why  can't  that  be  done  with 
lengths  ? 

M.  Tell  me  any  natural  measure  of  length. 

P. .  No,  I  am  afraid  I  can't.     But  why  is  there  a 

difference  ? 

M.  It  depends  on  the  fact  that  an  angle  cannot  be 
made  infinitely  great.  If  you  rotate  a  straight  line  round 
a  point  in  another  straight  line,  the  angle  between  both 
increases  at  first,  but  it  can't  become  larger  than  four 
right  angles,  for  that  angle  is  equal  to  the  angle  o,  and 
afterwards  the  same  angles  come  as  before.  The  largest 
possible  angle  has  consequently  a  finite  value,^  and  that 
value  is  the  natural  unit.  But  with  length  it  is  different, 
for  you  cannot  think  any  length  so  great  that  it  could 
not  be  made  greater. 

P.  So  nothing  which  can  be  made  infinitely  great  can 
have  a  natural  unit? 

M.  Quite  right.  You  will  soon  become  convinced 
that   for   all   such   magnitude   arbitrary   units   must   be 


MEASURING.  -  41 

chosen.  The  best  proof  is  that  no  one  has  been  able  to 
find  a  natural  one.  Now  we  will  go  back  to  the 
metre.  It  is  not  convenient  to  measure  all  magnitudes  of 
the  same  kind  by  means  of  the  same  unit.  You  can 
measure  the  length  of  the  table  in  centimetres;  but  if 
you  measure  the  height  of  a  hill  or  the  length  of  a  river 
in  centimetres,  your  numbers  will  be  far  too  large,  and 
for  such  great  lengths  larger  units  are  employed. 

P.  Yes,  I  know;    metres  and  kilometres. 

M,  Right.  People  have  used  such  different  units  for 
a  long  time,  but  they  did  not  stand  to  one  another  in  a 
sufficiently  simple  relationship.  At  the  same  time  as 
the  metre  was  introduced,  it  was  decided  only  to  admit 
such  measures  of  the  same  kind,  as  stand  to  each 
other  in  the  proportion  1:10:100:1000,  and  so  on;  that 
is,  in  powers  of  10. 

P.  Why  was  that  done  ? 

M.  Because  in  reducing  from  one  measure  to  another 
there  is  hardly  any  work  to  be  done ;  you  need  merely  add 
zeros,  or  alter  the  position  of  the  decimal  point.  Thus  you 
have: 

I  kilometre  (km.  for  short)  =  1000  metres  (m.  for  short). 

I  m.  =  io  decimetres  (dcm.)  =  ioo  centimetres  (cm.)  = 
1000  millimetres  (mm.). 

P.  What  is  the  meaning  of  kilo? 

M.  Kilo  is  the  Greek  word  for  a  thousand.  It  was 
agreed  at  the  same  time  that  the  multiple  of  each  unit 
should  be  expressed  by  Greek  prefixes  (deca-,  hecto-, 
and  kilo-) ;  while  the  fractions  are  expressed  with  Latin 
prefixes  (deci-,  centi-,  milli-). 

P.  Now  I  understand  the  meaning  of  the  words  kilo- 
gram and  milligram. 

M.  You  see  the  unit  of  mass  is    Jled  the  gram.     It  is 


42  CONFERS  A  TIONS  ON  CHE  MIS  TR  Y. 

derived  from  the  centimetre;  it  is  the  mass  of  a  cube  of 
water  at  4°  C.  The  multiples  deca-,  hecto-,  and  kilo- 
gram are  derived  from  it,  but  only  the  last  (kgrm.)  is  in  use. 
A  kilogram  is  equal  to  two  pounds.  The  deci-  and  centi- 
gram are  also  not  often  used;  but  the  milligram  (mgrm.)  = 
0.00 1  gram  is  much  used  in  scientific  work. 

P.  You  said  that  the  gram  is  the  unit  of  mass.  I  thought 
it  was  the  unit  of  weight,  for  people  weigh  with  grams 
and  kilograms. 

M.  Mass  and  weight  are  related  to  each  other.  Mass 
is  the  property  of  bodies  which  keeps  them  in  motion 
when  they  are  once  moving;  and  mass  is  measured  by 
the  work  which  must  be  expended  in  order  to  produce 
equal  velocities.  Now  weight  or  the  force  with  which 
bodies  are  drawn  to  the  earth  are  at  any  given  place 
exactly  proportional  to  the  mass,  so  that  when  two  weights 
are  equal  the  masses  are  also  equal.  And  for  that  reason 
masses  can  be  measured  by  help  of  weights. 

P.  Why  do  we  require  to  know  masses?  Surely  we 
buy  bread  and  iron  and  gold  by  weight. 

M.  Yes;  hy  weight,  but  not  on  account  of  weight.  In 
science  weight  is  derived  from  mass,  and  not  mass  from 
weight,  because  the  mass  of  any  body  is  unchangeable 
although  its  weight  may  be  altered. 

P.  But  if  I  keep  a  thing  carefully  shut  up,  so  that 
nothing  is  lost,  surely  its  weight  remains  unchanged? 

M.  I  don't  mean  it  in  that  sense.  Of  course,  if  you 
take  anything  away  from  a  body,  its  mass  will  be  de- 
creased in  the  same  proportion  as  its  weight.  No;  a 
body  has  a  smaller  weight  on  a  high  mountain  than  in  a 
valley.  And  weight  is  less  at  the  equator  than  at  the 
pole. 

P.  I  remember  learning  that  in  my  Geography  lesson; 


MEASURING.  43 

it  had  to  do  with  the  attraction  of  the  earth.  Because 
the  earth  is  flattened  at  the  poles,  a  body  there  is  nearer 
the  centre  of  the  earth  than  at  the  equator. 

M.  Quite  right;  but  you  must  add  that  the  attraction 
decreases  with  the  distance  from  the  centre  of  the  earth; 
moreover,  near  the  equator  the  centrifugal  force  increases, 
so  that  a  body  near  the  equator  is  more  swung  off  from 
the  earth  than  if  it  is  near  one  of  the  poles,  and  it  conse- 
quently weighs  less. 

P.  If  I  weigh  a  kilogram  of  sand  here,  and  carry  it  up 
a  high  mountain  and  weigh  it  again,  would  it  really  weigh 
less  ? 

M,  Not  if  you  were  to  weigh  it  on  an  ordinary  balance 
with  arms;  it  would  counterpoise  exactly  as  much  weight 
there  as  here. 

P.  But  you  said — 

M.  Don't  you  see  that  your  weights  become  lighter  in 
the  same  proportion  as  your  sand? 

P.  How  can  that  be?  Oh,  I  see;  I  hadn't  thought  of  it. 
But  I  can't  understand  how  it  can  be  proved  that  the  weight 
has  become  less. 

M.  By  determining  the  weight,  not  by  help  of  counter- 
poises, but  by  another  method.  A  spring  balance,  in 
which  weight  is  measured  by  stretching  a  spring,  would 
show  that  your  sand  weighed  less  on  the  top  of  a  hill 
\an  in  a  valley.     The  most  exact  measurement  is  made 

ith  a  pendulum,  for  it  swings  more  quickly  the  greater 
ihe  attraction. 

P.  Why? 

M.  You  will  learn  it  in  your  Physics  lesson.  We  must 
go  back  to  our  old  subject.  I  told  you  that  things  are 
bought  by  weight,  not  because  of  weight.  Why  do  people 
buy  bread? 


44  COr^VERSATIONS  ON  CHEMISTRY. 

P.  To  eat  it. 

M.  Do  you  eat  it  in  order  to  grow  heavier? 

P.  Ha!  hal  ha!  No,  because  I  Hke  it  and  because 
it  makes  me  strong. 

M\  The  last  is  the  important  reason.  And  coals  are 
bought,  not  because  they  are  heavy,  but  because  they 
make  you  warm. 

P.  But  I  can't  understand  the  use  of  weight. 

M.  Which  would  you  rather  have,  a  small  piece  of  cake 
or  a  large  one? 

P.  Of  course,  a  large  one. 

M.  Why? 

P.  Because  there  is  more  of  it.  A  little  one  wouldn't 
satisfy  my  hunger. 

M.  And  which  weighs  more  ? 

P.  The  larger  one,  of  course. 

M.  Now  you  see  the  use  of  weight.  The  properties 
and  uses  which  make  us  buy  things  increase  or  decrease 
with  the  mass  or  the  weight.  The  power  which  bread 
has  of  keeping  you  alive  increases  proportionally  to  its 
weight,  and  the  greater  the  weight  of  the  coal  you  buy 
the  more  heat  you  can  get  from  it,  and  just  as  with  these 
marketable  properties,  so  a  great  many  scientific  prop- 
erties are  dependent  on  the  mass  and  on  the  weight 
The  balance  is  therefore  a  very  important  piece  of  chemical 
apparatus,  not  so  much  because  we  want  to  know  the 
weight  of  things,  for  often  we  do  not  care  to  know  it,  but 
because  of  the  other  properties  which  are  connected  with 
weight. 

P.  So  weight  is  like  the  paper  of  a  book,  which  is 
worth  very  little  in  itself,  but  becomes  valuable  for  what 
is  printed  on  it. 

M.  That  is  a  good  comparison  even  though  it  doesn't 


MEASURING.  45 

quite  fit.  Let  us  take  a  better  example.  As  you  know, 
liquids  are  bought  and  sold  both  by  measure  and  weight. 
Wine  and  beer  are  sold  only  by  measure,  that  is,  by  the 
space  which  they  occupy;  paraffin-oil  is  sold  both  by 
weight  and  by  measure;  sulphuric  acid  is  sold  only  by 
weight. 

P.  Why? 

M.  Convenience  and  custom  are  the  reasons.  Measur- 
ing is  much  quicker  than  weighing,  and  a  measure  is 
much  more  easily  made  than  a  balance;  and  so  this  plan  is 
preferred.  But  sulphuric  acid  is  a  somewhat  dangerous 
liquid,  and  people  don't  like  to  pour  it;  therefore  they 
prefer  to  weigh  it.  But  for  the  purpose  of  determining 
quantity  by  measurement,  for  any  one  substance  volume 
and  weight  bear  a  constant  proportion  to  each  other. 
Hence  the  actions  and  uses  of  liquids  are  proportional 
to  their  volumes,  just  as  they  are  to  their  weights.  The 
purchaser  of  paraffin-oil  is  not  interested  in  the  volume 
it  occupies  or  in  its  weight;  he  buys  it  because  of  the 
amount  of  light  or  heat  which  he  can  get  from  it.  But 
these  amounts  are  proportional  to  the  volume,  and  so 
the  volume  becomes  a  measure  for  the  amount  of  light 
which  the  paraffin-oil  will  produce.  Now  tell  me  what 
you  know  about  measures  of  volume. 

P.  The  unit  is  called  a  litre. 

M.  That  is  only  half  right.  The  real  unit  of  volume  is 
derived  from  the  unit  of  length,  and  is  a  cube,  the  side  of 
which  is  one  metre  long — a  cubic  metre.  But  this  measure 
is  far  too  large  for  most  purposes,  and  therefore  one  has 
been  chosen  nearer  in  volume  to  the  old  pints  and  gallons. 
It  is  a  cube,  the  side  of  which  is.  Yio  of  a  metre;  its 
capacity  therefore  is  Yiooo  of  ^  cubic  metre.  It  is  called  a 
cubic  decimetre,  or  a  litre  (1.). 


46  CONyERS/l  TIONS  ON  CHE  MIS  TR  Y. 

P.  You  have  surely  made  a  mistake  in  saying  that  a 
cubic  decimetre  is  a  thousandth  of  a  cubic  metre.  A  deci- 
metre is  only  a  tenth  of  a  metre. 

M.  Think  a  minute! 

P.  What  a  stupid  I  was!  The  volume  of  a  body  is  pro- 
portional to  the  cube  of  its  side,  and  10X10X10=  1000. 

M.  Yes,  that  is  right.  In  science  we  use  as  a  measure 
one-thousandth  of  a  litre.     How  large  is  that  cube? 

P.  I  won't  make  another  mistake.  The  side  is  ten 
times  less,     y^o  dcm.  is  Yioo  i^i.     It  is  a  centimetre. 

M.  The  measure  of  volume  is  called  cubic  centimetre 
(ccm.).  Now  write  me  down  a  table  of  measures  of 
volumes. 

P.  I  cbm.  =  1000  ].,  and  i  1.  =  1000  ccm. 

M.  Quite  right.  Now  we  have  had  enough  for  to-day, 
although  there  is  a  great  deal  more  to  say  about  measure- 
ment. 


8.  DENSITY. 

M,  Yesterday  you  learned  how  to  measure  and  to 
weigh;  to-day  we  will  talk  a  little  more  about  measure- 
ment. Which  is  the  lighter,  a  pound  of  lead  or  a  pound 
of  feathers? 

P.  You  can't  catch  me  with  that  old  joke.  Of  course 
they  are  the  same  weight. 

M.  But  which  is  the  lighter,  lead  or  feathers? 

P.  Hm!     Well,  feathers  are  really  lighter. 

M.  That  is  a  contradiction.  It  depends  upon  the  fact 
that  the  words  light  and  heavy  are  used  with  a  double 
meaning.  When  you  say  lead  is  heavier  than  feathers, 
you  mean  that  a  handful  of  lead  has  a  greater  weight 


DENSITY.  47 

than  a  handful  of  feathers;  if  equal  volumes  of  feathers 
and  of  lead  are  compared,  the  lead  weighs  more.  If  we 
say  wood  is  lighter  than  iron,  we  attach  the  same  meaning 
to  the  word  lighter,  although  you  could  easily  choose  a 
given  piece  of  wood  heavier  than  a  given  piece  of  iron. 

P.  I  understand  that. 

M.  But  in  science  it  doesn't  do  to  use  such  indefinite 
expressions.  The  property  which  is  greater  with  iron 
and  lead  than  with  wood  and  feathers  is  called  density^ 
and  we  say  iron  is  denser  than  wood  and  lead  denser  than 
feathers.     How  is  density  determined? 

P.  By  weight  and  by  volume. 

M.  Yes.  And  as  the  density  is  greater  the  greater  the 
weight  in  a  given  volume,  and  smaller,  the  greater  the 
volume  of  a  given  weight,  the  density  is  made  propor- 
tional to  the  weight  and  inversely  proportional  to  the 
volume;  so  that  if  w  is  the  weight  and  v  the  volume,  the 
density  d  is  expressed  by  the  formula 


d=~, 

V 


P.  What  is  the  use  of  this  formula  ? 

M.  To  measure  the  density.  Let  us  take  an  example: 
What  is  the  density  of  water? 

P.  It  depends  on  what  weight  and  what  volume  you 
take. 

M.  No,  it  doesn't  depend  upon  that.  We  choose  once 
for  all  the  gram  as  unit  of  weight  and  the  cubic  centimetre 
as  unit  of  volume.  Now,  if  we  take  an  arbitrary  quantity 
of  water,  say  a  litre,  what  is  its  weight  ? 

P.  One  litre  of  water  weighs  looo  grams. 

M.  And  what  is  its  volume  in  cubic  centimetres? 


48  CONVERSATIONS  ON  CHEMISTRY. 

P.  looo  c.c.  make  a  litre. 

M.  So  we  have  w=  looo  and  v  =  looo;  how  large  is  </? 

P.  J=  1000/1000=1;    the  density  is  i. 

M.  Now  make  the  same  calculation  for  20  c.c.  of 
water. 

P.  ^^=20/20=1.  It  is  I  again.  Oh,  I  see;  because 
the  volume  and  the  weight  always  become  larger  and 
smaller  to  the  same  degree,  the  fraction  must  always  have 
the  same  value  whatever  quantity  of  water  is  taken. 

M.  Now  you  understand  it.  Here  I  have  a  little  lead 
cube;  what  is  its  density? 

P.  I  must  first  find  its  weight.  Let  me  weigh  it  myself. 
It  weighs  38.84  grams.  And  now  I  must  find  its  volume. 
But  how  can  I  do  that? 

M.  As  it  is  a  cube  you  have  only  to  determine  the 
length  of  one  side.     Here  is  a  rule. 

P,  The  side  is   15   mm.  long,  and  so  the  volume  is 

i5'  =  3375- 

M.  Equals  3375  what? 

P.  3375  c.mm.  Oh,  I  should  give  the  volume  in 
cubic  centimetres.  I'll  be  right  this  time.  The  volume 
is  3.375  cc.  % 

M.  Quite  right.     Now  calculate  the  density. 

P.  38.84/3.375  =  11.51- 

M.  So  the  cube  has  the  density  11.51.  I  can  go  further 
and  say  that  lead  has  the  density  11.51,  for  if  I  had 
taken  any  other  cube  of  lead,  or  indeed  any  other  piece  of 
lead,  I  should  have  found  the  same  number.  Tell  me 
why. 

P.  I  can  see  that  you  would  have  got  about  the  same 
number,  but  I  am  not  sure  that  you  would  have  got  exactly 
the  same  number. 

M'  You  have  forgotten  what  I  told  you  before  (page  2) 


DENSITY.  49 

about  properties.  Density  is  a  property;  for  all  samples 
of  the  same  substance  it  will  have  the  same  value.  Now 
ordinary  lead  is  really  a  very  pure  substance,  and  con- 
tains hardly  anything  mixed  with  it,  and  so  the  properties 
of  different  samples  have  the  same  value. 

P.  But  all  bodies  expand  with  heat ;  so  that  the  volume 
of  the  lead  cube  will  be  larger  when  it  is  warm  than  when 
it  is  cold. 

M.  Quite  right.     Is  weight  changed  by  heat  ? 

P.  Not  so  far  as  I  know. 

M.  Weight  is  quite  independent  of  temperature.  So 
it  follows  that  the  density  of  lead  becomes  smaller  as  the 
temperature  rises,  because  while  the  nunierator  remains 
the  same,  the  denominator  increases. 

P.  Then  density  isn't  quite  a  definite  property. 

M.  Yes,  it  is,  for  at  a  definite  temperature  it  has  a 
definite  value.  The  same  holds  for  every  other  sub- 
stance. Water,  too,  changes  its  volume  with  temperature; 
and  therefore  4°  has  been  chosen  as  the  temperature  at 
which  the  weight  of  i  c.c.  is  called  i  gram. 

P.  Why  was  that  temperature  chosen? 

M.  Because  water  has  its  greatest  density  or  its  smallest 
volume  at  4°.     What  are  you  thinking  about? 

P.  I  am  thinking  how  it  would  be  possible  to  determine 
the  density  if  the  thing  wasn't  a  cube. 

M.  That  is  a  very  sensible  question,  for  very  few  sub- 
stances can  be  made  into  that  shape.  Look  here,  I'll 
show  you  how  it  can  be  done.  Here  is  a  glass  tube 
which  is  divided  into  tenths  of  cubic  centimetres  by 
little  lines.  I  pour  water  into  it  and  read  where  the 
level  stands;    I  find  5.33  c.c. 

P.  You  have  read  off  hundredths,  and  there  are  only 
tenths  marked  upon  the  tube. 


50  CONVERSATIONS  ON  CHEMISTRY. 

M.  Every  one  who  makes  measurements  must  learn  to 
do  that.  As  a  rule,  the  level  of  the  water  does  not  lie 
neatly  on  a  line,  but  between  two.  I  divide  the  distance 
between  two  lines  into  tenths  with  my  eye,  and  so  I  get  my 
hundredths. 

P.  I  couldn't  do  that. 

M.  It  isn't  difficult  to  learn,  and  you  must  try  it  after- 
wards. But  now  we  will  go  on.  I  have  here  a  glass 
with  shot.     They  are  made  of  lead;  weigh  it. 

P.  It  weighs  43.58  grams. 

M,  Now  I  shake  some  of  the  shot  into  the  tube. 
Weigh  the  glass  again. 

P.  It  weighs  28.42  grams. 

M,  What  is  the  weight  of  the  shot  that  I  have  shaken 
into  the  tube? 

P.  43.58-28.42  =  15.16  grams. 

M,  And  now  I  read  the  level  of  the  water  in  the  tube. 
It  stands  at  6.66.  That  is  1.33  c.c  more.  What  con- 
clusion can  I  draw? 

P.  Oh,  now  I  see.  The  volume  of  the  water  has  risen 
so  as  to  tell  the  volume  of  the  shot.  The  volume  of  15.16 
grams  is  1.33  c.c,  and  so  its  density  is  11.40.  It  is  almost 
exactly  the  same  number  that  we  calculated  before. 
But  it  is  not  exactly  right. 

M.  Because  you  didn't  measure  with  sufficient  accu- 
racy. You  gave  the  side  of  the  cube  as  15  mm.;  measure 
again. 

P.  Yes,  it  is  a  little  smaller. 

M.  And  measure  the  other  sides  of  the  cube. 

P.  They  are  not  quite  equal. 

M.  You  see,  then,  that  your  former  measurement  con- 
tained errors,  and  therefore  the  result  cannot  be  quite 
accurate.     To  measure  exactly  is  a  very  difficult  thing; 


DENSITY.  51 

and  therefore  we  must  rest  contented  at  present  with 
what  we  have  found;  the  right  number  is  11.4.  I  will 
let  you  use  the  balance  and  the  measuring-glass,  and  you 
can  determine  for  yourself  the  density  of  various  sub- 
stances. But  take  care  that  you  always  remove  the 
bubbles  of  air,  or  you  will  measure  them  along  with 
the  volume  of  the  body,  which  will  appear  too  great, 
and  you  will  get  too  small  densities. 

P.  Yes,  I  will  draw  up  a  table.  What  shall  I  meas- 
ure? 

M.  You  had  better  find  the  densities  of  your  minerals. 
But  now  to  another  question:  Have  liquids  also  definite 
densities  ? 

P,  I  think  so.     Yes,  water  has  the  density  i. 

M.  Right.  Now  think;  how  can  you  determine  the 
density  of  a  liquid? 

P.  By  determining  its  weight  and  its  volume.  Wait, 
I  know.  I  shall  pour  it  into  the  measuring-glass  and 
read  out  its  volume. 

if.  And  how  will  you  find  its  weight? 

P.  Exactly  as  you  did  with  the  shot.  I  shall  first 
weigh  the  flask  which  contains  the  liquid,  then  pour  it 
into  the  measuring-glass,  and  then  I  shall  weigh  the  flask 
again. 

M.  It  can  be  done  in  that  way,  but  it  is  possible  to  do 
it  in  a  much  simpler  manner.  Weigh  the  measuring- 
glass  once  for  all,  then  pour  in  liquid  and  weigh  again, 
and  you  need  only  subtract  the  weight  of  the  measuring- 
glass. 

P.  That  gives  me  one  weighing  less. 

M.  You  can  lessen  your  work  still  further  by  not 
measuring  out  an  arbitrary  quantity  of  liquid,  but  a 
definite   volume.     This   is   not   easy   with   solid  bodies, 


52  CONVERS/iTIONS  ON  CHEMISTRY. 

but  is  quite  easy  with  liquids,  because  they  fill  a 
given  volume  completely.  For  example,  if  you 
pour  exactly  i  c.c.  into  your  measuring-glass, 
and  determine  its  weight,  what  will  your  equa- 
tion be? 

P.  Then  d^g/i.  That  i?>  d= g\  the  weight 
is  the  same  as  the  density. 

M.  Do  you  see  you  don't  require  to  divide. 
It  is  often  said  that  the  density  is  the  weight  of 
unit  volume.  This  expression  is  not  wrong, 
but  doesn't  cover  enough,  and  so  I  didn't  tell 
you  it  before. 

P.  I  have  just  tried  to  pour  i  c.c.  of  water 
into  the  measuring-glass,  but  it  is  very  difficult 
to  get  exactly  the  right  amount.  I  have  found 
either  too  much  or  too  little. 

M.  Pour  in  a  little  too  much,  and  then  remove 
the  excess  with  a  small  strip  of  blotting-paper. 
It  sucks  up  such  small  quantities  that  it  is  quite 
easy  to  obtain  the  right  volume. 
FiG."io.       P.  Yes,  that  works. 

M,  It  is  still  easier  with  this  apparatus  (Fig.  lo),  which 
is  called  a  pipette  (this  is  a  French  word  and  means  little 
pipe).  I  suck  the  upper  end  while  I  hold  the  lower  in 
the  liquid,  until  the  level  rises  above  a  mark  on  the  stem; 
then  I  close  the  end  quickly  with  my  forefinger,  and  while 
the  point  touches  the  side  of  the  vessel,  I  can  easily 
let  so  much  liquid  run  out  that  it  stands  exactly  at  the 
mark. 

P.  But  I  must  put  the  liquid  in  another  vessel  to  weigh 
it. 

M.  No.  You  can  lay  the  pipette  itself  upon  the  scale. 
If  you  have  determined  its  weight,  when  empty  once  for  all, 


DENSITY.  53 

you  need  only  subtract  that  from  the  total  weight  and  you 
have  the  weight  of  a  cubic  centimetre,  or  the  density.  It 
is  still  simpler  to  make  a  counterpoise  of  wire  of  the  same 
weight  as  the  pipette.  Such  a  counterpoise  is  called  a 
tare.  Then  the  remainder  of  the  weights  on  the  pan 
will  give  you  the  density. 

P.  I'll  certainly  do  that. 

M.  In  that  manner  you  can  determine  the  densities 
of  various  liquids,  such  as  spirits  and  salt  water.  You 
will  find  the  first  lighter,  the  second  heavier,  than 
water. 

P.  Then  I  can  make  a  table  of  densities  of  liquids 
as  well. 

M.  Now  you  know  how  to  determine  densities  of  solids 
and  liquids,  what  about  gases? 

P.  Can't  their  densities  be  determined  in  the  same 
way  by  measuring  their  weight  and  their  volume? 

M.  Of  course  they  can,  but  it  is  not  so  easy.  In  the 
first  place  the  weight  of  a  large  volume  of  air  is  very 
small;  i  litre  of  air  weighs  only  a  little  more  than  i 
gram,  as  you  have  seen  already.  Then  the  volume  of 
gases  is  very  easily  changed  if  the  temperature  or  the 
pressure  alters.  And  so  very  different  densities  are  got 
for  the  same  gas  if  it  is  measured  at  different  temperatures 
or  pressures. 

P.  But  that  happens  too  with  solids  and  liquids. 

M.  The  changes  are  much  smaller  with  them,  so  that 
they  need  only  be  taken  into  account  if  great  accuracy 
is  required. 

P.  Then  how  is  the  density  of  a  gas  determined  ? 

M.  That  is  a  rather  difficult  thing,  which  I  shall  explain 
to  you  later.  To-day  I  will  merely  say  that  people  have 
determined  upon  a  standard  temperature  and  a  standard 


54  CON  VERSA  TJONS  ON  CHE  MIS  TR  Y. 

pressure  at  which  to  measure  the  volumes  of  gases,  and 
so  uniform  results  are  obtained. 

P.  I  should  never  have  thought  that  measuring  was 
such  a  difficult  matter. 


9.  FORMS. 

M.  I  am  not  going  over  to-day  what  you  learned  yes- 
terday, because  it  was  really  just  a  rep^titioi  of  what  you 
had  learned  before.  We  will  go  back  to  what  we  spoke 
about  in  the  lesson  before  last.  You  learned  two  very 
different  properties  of  water.  What  law  is  at  the  bottom 
of  the  melting  of  ice  and  boiling  of  water? 

P.  That  both  happen  at  a  definite  temperature. 

M.  Yes,  but  not  only  water;  every  substance  has  these 
properties. 

P.  Really  all? 

M.  All  substances  that  are  really  pure  substances. 
Mixtures  and  solutions  have  changeable  melting-  and 
boiling-points. 

P.  How  changeable? 

M.  If  a  solution  is  brought  to  boiling  point,  we  notice, 
as  the  boiling  proceeds,  that  the  temperature  doesn't  re- 
main unchanged,  as  with  pure  substances,  but  gradually 
rises,  in  proportion  to  the  amount  of  steam  that  goes 
away.  In  the  same  way,  when  a  mixture  fuses  or  melts, 
it  begins  to  liquefy  at  a  definite  temperature;  this  does 
not  remain  stationary,  however,  but  rises  higher  as  more 
heat  is  added  and  more  of  the  mixture  becomes  liquid. 

P.  May  I  see  that  ? 

M.  Later.  At  present  we  will  stick  to  pure  sub- 
stances.   You  have  seen  that  liquid  water  can  be  changed 


FORMS,  55 

into  solid  ice  and  into  gaseous  steam.  Do  you  know 
what  these  two  conditions  are  called? 

P.  Yes;  states  of  aggregation. 

M.  Quite  right;  that  is  the  usual  name.  What  does 
it  mean? 

P.  Aggregate  means  assembled,  but  I  don't  know  what 
that  has  to  do  with  liquid  or  steam. 

M.  The  name  is  given  because  it  is  taken  for  granted 
that  all  bodies  are  made  up  of  tiny  particles  which  are  able 
to  lie  on  each  other,  or  arrange  themselves  in  various 
ways.  They  are  called  atoms.  According  as  these 
atoms  are  near  or  far  from  each  other,  they  make  solid, 
liquid,  or  gaseous  bodies. 

P.  Can  you  see  these  atoms  with  a  glass? 

M.  No,  not  even  with  the  strongest  microscope. 
People  take  for  granted,  because  of  that,  that  they  are 
•smaller  than  the  smallest  thing  that  can  be  seen  through 
a  microscope. 

P.  But  are  they  really  there  ? 

M.  It  is  true  I  cannot  guarantee  them.  There  is  no 
proof  of  their  existence. 

P.  Then  how  can  you  say  that  it  depends  upon  them 
whether  a  body  is  solid  or  liquid  ? 

M.  Real  things  behave  in  many  respects  as  if  they 
were  collections  of  atoms,  if  atoms  exist.  If  it  be  assumed 
that  bodies  consist  of  atoms,  it  may  be  deduced  that  they 
must  behave  as  they  really  do. 

P.  That  is  very  awkward.  Why  do  people  not  simply 
say:  They  behave  this  way  or  that  way,  and  be  done 
with  it  ? 

M,  Because,  starting  with  the  assumption  of  atoms, 
there  can  be  deduced  several  conclusions  which  agree 
with  fact.     Such  an  assumption  is  called  a  hypothesis. 


S6  CONVERSATIONS  ON  CHEMISTRY. 

P.  But  I  can't  see  what  is  gained  if  there  is  no  proof 
that  the  hypothesis  is  true. 

M.  The  hypothesis  serves  to  make  the  real  relation- 
ship more  easily  noticeable.  If  you  have  to  keep  in  mind 
three  names,  Alfred,  Arthur,  and  Anthony,  it  will  be 
easier  for  you  to  remember  them  if  you  notice  that  they 
all  begin  with  A.  Moreover,  a  hypothesis  serves  as  a 
stimulus  to  research.  People  imagine  how  a  number  of 
atoms  would  behave  under  given  circumstances,  and 
find  out  whether  the  actual  bodies  behave  in  that  way. 

P.  Do  they  always  agree? 

M.  No,  I  am  sorry  to  say,  not  always. 

P.  But  after  such  a  conclusion  has  been  drawn,  people 
ought  to  see  whether  it  is  right  or  not. 

M.  Certainly;  but  this  gives  an  opportunity  of  putting 
definite  questions  to  nature  and  of  making  suitable 
experiments  or  observations.  And  so  our  knowledge 
increases,  and  that  is  always  an  advantage. 

P.  But  if  they  don't  agree? 

M.  Then  there  is  nothing  for  it  but  to  wait  and  hope 
that  the  contradiction  may  be  explained. 

P.  But  that  is  a  very  uncertain  way  of  doing  things. 

M.  So  it  is;  yet  the  use  of  hypotheses  for  learning 
and  investigation  is  so  great  that  people  will  always 
make  use  of  them. 

P.  Couldn't  they  do  without  them? 

M.  Of  course  they  could;  but  people  are  so  much  in 
the  habit  of  using  hypotheses,  like  the  atomic  hypothesis, 
that  they  find  great  inconvenience  when  they  try  to  realize 
things  without  their  help.  And  therefore  they  will  not 
give  them  up. 

P.  Then  please  explain  to  me  how  solid,  liquid,  and 
gaseous  bodies  are  built  up  of  atoms. 


FORMS.  57 

M.  Ah,  you  put  me  in  a  difficult  position  if  you  wish 
me  to  show  you  the  use  of  the  atomic  hypothesis,  for 
up  to  the  present  it  has  not  been  entirely  satisfactory. 
However,  we  needn't  delay  over  that  at  present;  I  only 
mentioned  the  subject  in  order  to  explain  the  derivation 
of  the  name  "state  of  aggregation."  In  talking  over  these 
things  with  you  I  prefer  to  consider  these  relations  without 
its  help;  and  for  that  reason  I  will  not  use  the  term,  but 
rather  speak  of  forms. 

P.  What  does  the  name  mean? 

M.  It  points  to  the  chief  differences  of  these  states. 
How  does  a  solid  body  behave  in  relation  to  its 
form? 

P.  I  don't  know  anything  particular  to  say  about  that; 
it  can  be  broken,  cut,  or  bent. 

M.  But  if  it  is  left  alone  ? 

P.  Then  it  keeps  its  form. 

M.  Right.  Have  you  ever  thought  how  important 
that  is? 

P.  I  don't  see  anything  very  important  about  it. 
Sometimes  it  is  a  great  nuisance;  for  example,  if  I  want 
to  break  sugar. 

M.  Think  for  a  minute.  If  the  stones  and  rafters  of 
this  house  were  to  change  their  shape,  it  might  fall  to 
pieces  at  any  moment;  none  of  our  tools  would  be  usable; 
you  couldn't  cut  .with  a  knife  if  the  blade  didn't  keep  its 
shape;  your  morning  milk  wouldn't  stay  in  its  can  if  the 
shape  of  the  can  kept  changing  continually. 

P.  Yes,  now  I  see,  but  I  can't  think  it  out  to  the  end. 
The  whole  world  would  go  to  bits. 

M.  Now  I  see  you  are  beginning  to  grasp  it.  Have  all 
bodies  the  property  of  keeping  their  shape  ?  For  instance, 
how  does  water  behave  in  this  respect? 


58  CONVERSATIONS  ON  CHEMISTRY. 

P.  Water  does  not  keep  its  shape;  you  may  pour  it  into 
any  sort  of  vessel  you  like. 

M.  Is  water  the  only  thing  that  has  this  property  ? 

P.  No,  all  Hquid  bodies  are  like  it.  Yes,  now  I  see  the 
great  difference.  But  how  is  it  that  solid  bodies  keep 
their  shape? 

M.  That  is  a  senseless  question.  How  do  you  know 
when  a  body  is  solid? 

P.  I  catch  hold  of  it. 

M.  And  you  are  convinced  that  it  keeps  its  shape. 
The  word  solid  is  merely  the  name  for  the  common 
properties  of  many  bodies  of  keeping  their  shape. 

P.  But  that  must  have  a  cause. 

M.  I  don't  understand  you. 

P.  Why  is  this  silver  coin  not  liquid  ? 

M.  Well,  when  you  heat  it,  it  melts  and  becomes 
liquid.  Here  is  a  piece  of  thin  silver  gauze;  if  I  hold  it 
in  the  flame  it  will  liquefy,  and  a  drop  will  form  on  the 
end.     See,  the  drop  has  fallen. 

P.  So  it  has. 

M.  The  question  whether  a  body  is  solid  or  liquid 
depends  solely  upon  its  temperature.  Below  its  melting- 
point  it  is  a  solid,  and  above  its  melting-point  it  is  a  liquid. 

P.  Is  that  the  case  with  all  bodies  ? 

M.  Yes. 

P.  Then,  by  cooHng,  any  liquid  can  be  made  solid, 
and  all  solids  become  liquid  when  heated? 

M.  Generally.  If  substances  do  not  decompose  they 
behave  in  that  manner.  Only  there  are  liquids  the  freez- 
ing-point of  which  is  very  low,  and  solids  which  melt  at  a 
very  high  temperature.  There  are  melting-  and  freezing- 
points  in  all  regions  of  temperature. 

P.  Why  does  a  solid  freeze  at  a  definite  temperature? 


FORMS.  59 

M.  That  is  another  senseless  question.  You  can  only 
ask :  What  is  the  freezing-point  connected  with  ?  It  is 
just  as  if  you  were  to  ask:  Why  are  there  camels? 
Whereas  one  can  only  ask:  What  properties  have  these 
animals,  and  how  do  these  properties  compare  with  those 
of  other  animals?  In  the  same  kind  of  way,  melting- 
points  are  phenomena  of  nature,  and  have  definite  rela- 
tions to  other  phenomena. 

P.  What  sort  of  relations? 

M.  If  I  were  to  answer  that  question  you  wouldn't 
understand,  for  you  would  first  have  to  know  those 
other  properties. 

P.  Yes,  that  is  true.  I  see  you  would  require  to 
know  the  other  properties  before  you  could  compare 
relations  between  them. 

M.  Yes.  So  we  must  begin  our  work  by  collecting 
facts,  by  writing  them  down,  and  then  by  comparing 
them  with  each  other  in  order  to  find  out  in  what  they 
agree.     That  is  how  laws  of  nature  are  discovered. 

P.  I  never  thought  of  it  in  that  way.  I  supposed 
that  some  very  clever  man  must  have  discovered  them 
all  by  himself. 

M.  Nobody  does  anything  all  by  himself,  as  you 
call  it.  But  think  a  minute.  One  law  of  nature  tells 
us  how  certain  things  will  behave  under  definite  condi- 
tions. The  thing  must  be  known  under  those  condi- 
tions before  such  statements  can  be  made. 

P.  Yes,  that  is  so ;  but  then  every  one  must  be  able  to 
discover  laws  of  nature. 

M.  And  so  any  one  can,  if  he  finds  things  in  conditions 
which  have  not  yet  been  sufficiently  investigated.  But 
that  is  rather  difficult,  because  the  common  and  ordinary 
conditions  of  things  are  already  discovered ;  and  it  is  very 


6o  CONyERSATIONS  ON  CHEMISTRY. 

hard  to  acquire  enough  exact  knowledge  to  find  undis- 
covered spheres  to  examine.  For  instance,  it  would  be 
quite  easy  to  discover  the  north  pole  if  you  could  only 
get  to  it.  The  difficulty  is  not  to  see  the  north  pole,  but 
to  get  a  place  from  which  it  can  be  seen. 

P.  Then  I  will  really  learn  thoroughly,  and  perhaps 
later  I  may  discover  something. 

M.  Yes,  do  so.  You  know  the  end  in  view,  anyhow. 
But  now  we  will  return  to  our  subject.  Do  you  under- 
stand now  the  meaning  of  the  name  forms? 

P.  Yes,  solids  have  forms,  but  liquids  haven't. 

M.  That  is  partly  right;   but  what  about  gases? 

P.  They  haven't  any  form,  either. 

M.  How  do  they  differ  from  liquids? 

P.  They  are  far  lighter  and  thinner. 

M.  Yes,  that  is  right,  but  you  haven't  come  to  the  main 
point.  If  I  pour  some  liquid  into  a  vessel,  it  falls  to  the 
bottom,  and  fills  the  vessel  according  to  the  amount. 
But  if  I  put  gas  in  an  empty  vessel,  what  happens 
then? 

P.  I  don't  know;   a  gas  can't  be  seen. 

M.  It  fills  the  whole  vessel,  however  much  or  httle 
there  is. 

P.  That  is  extraordinary.     How  do  you  know  that? 

M.  Only  a  definite  amount  of  any  sort  of  liquid  can 
be  poured  into  a  given  vessel,  that  is,  as  much  as  there  is 
room  for.     If  less  is  put  in — 

P.  A  part  of  the  vessel  remains  empty. 

M.  Right.  If  you  attempt  to  put  more  in,  it  doesn't 
work,  for  a  liquid  doesn't  allow  itself  to  be  pressed  together, 
or,  to  be  exact,  only  slightly.  A  great  quantity  of  gas  can 
be  put  into  a  given  space,  and  it  is  always  possible  to  put 
in  still  more. 


COMBUSTION,  6l 

P.  Does  that  go  on  forever? 

M.  No;  more  and  more  pressure  is  needed  for  it. 
We  shall  soon  go  into  these  things  more  particularly. 
At  present  the  difference  between  liquid  and  gaseous 
bodies  is  important  to  us.  It  is  true  that  liquids  have 
no  definite  form,  but  they  have  a  definite  space  or  vol- 
ume, which  is  unchangeable  whatever  form  they  may  be 
made  to  take.  So  a  litre  of  petroleum  is  always  a  litre, 
whether  it  is  in  a  can  or  a  jug,  or  anything  else  it  may 
be  kept  in. 

P.  And  gases? 

M.  Gases  have  neither  a  definite  form  nor  a  definite 
volume,  but  spread  themselves  out  through  all  the  avail- 
able space  until  they  entirely  fill  it. 

P.  Then  the  name  "form"  isn't  suitable  for  gases? 

M.  Not  at  all.  Liquids  take  the  form  of  whatever 
contains  them,  but  only  so  far  as  they  fill  it.  Gases 
take  the  form  of  whatever  contains  them,  because  they 
always  fill  it  completely. 

P.  Then  "form"  is  the  way  in  which  bodies  take 
their  form! 

M.   You  may  describe  the  word  in  that  way. 

10.  COMBUSTION, 

M.  Now  you  know  something  more  about  all 
the  three  states,  and  can  have  a  more  complete  idea 
that  nearly  all  substances  are  known  in  these  three 
forms. 

P.  Why  not  all? 

M.  With  some  the  melting-  or  boiling-point  is  so  high, 
or  the  freezing-point  so  low,  that  it  is  not  possible  to 
reach  them, 


62  CONVERSATIOl^S  ON  CHEMISTRY, 

P.  Oh,  I  wanted  to  ask  you  about  that  a  long  time 
ago :  are  these  changes  of  one  form  into  another  chemical 
or  physical  reactions  ? 

M.  You  know  that  such  a  classification  is  more  or 
less  arbitrary.  If  we  define  a  chemical  process  as  one 
in  which  most  of  the  properties  of  the  substances  con- 
cerned alter,  then  we  must  consistently  define  change  of 
state  as  involving  a  chemical  process. 

P.  But  we  spoke  about  melting  and  boiling  in  my 
Physics  lesson,  so  they  belong  to  physics. 

M.  Ice  can  be  changed  as  easily  into  water  as  water 
into  ice.  But  in  chemical  changes,  only  one  change, 
generally,  is  easily  made;  the  other  causes  great  diffi- 
culty. Formerly,  because  of  this  difference,  people  did 
not  consider  change  of  form  as  a  chemical  change. 

P.  You  said  "formerly";  is  it  different  now? 

M.  Now  people  have  learnt  that  many  changes  which 
are  generally  called  chemical  can  be  reversed,  and  are 
subject  to  the  same  laws  as  physical  changes. — But 
now  we  will  turn  to  things  which  have  always  been 
looked  upon  as  chemical.  Have  you  ever  looked  at  a 
candle  burning?  Yes?  Then  describe  to  me  what  you 
saw. 

P.  When  you  light  a  candle  it  burns  down  till  it  is  all 
gone,  and  during  this  it  has  a  hot,  bright  flame. 

M.  Right.     What  is  necessary  for  burning? 

P.  Well,  the  candle. 

M.  Nothing  else? 

P.  Not  that  I  know  of. 

M.  If  you  put  the  burning  candle  in  water — 

P.  It  goes  out. 

M.  Why?     What  is  different  from  before? 

P,  It  has  no  more  air, 


COMBUSTION, 


63 


M.  Right.  For  burning,  then,  it  is  necessary  to  have 
the  candle  and  air.  I  will  show  you  now  that  a  candle 
can  burn  under  water  if  it  is 
only  put  under  together  with  air. 
I  let  a  little  board  float  about  on 
the  water  in  this  large  glass, 
place  a  bit  of  burning  candle 
on  it,  turn  a  glass  upside  down 
over  it,  and  now  I  can  dip  the 
whole  thing  under;  the  candle 
is  burning  (Fig.  11). 

P.  Oh,  that  is  pretty!  Please 
hold  it  a  little  longer  like  that. 
Ah!  the  flame  has  gone  out. 
Some  water  must  have  got  on 
the  wick. 

M.  We  will  make  the  experi- 
ment again,  and  hold  the  glass  quite  still. 

P.  The  flame  went  out  again,  after  it  had  burnt  a 
little. 

M.  Now  we  will  leave  out  the  water  altogether.  I 
put  the  little  candle  on  a  smooth  plate  of  glass  and  place 
a  glass  beaker  firmly  over  it. 

P.  The  flame  is  going  out  again. 

M.  What-  must  you  conclude  from  this  experiment  ? 

P.  That  the  candle  can't  burn  for  long  in  a  glass 
beaker. 

M.  That  would  not  be  right.  I  put  my  beaker  up- 
right, and  put  the  candle  in.  You  see  it  burns,  perhaps 
a  little  unsteadily,  but  it  doesn't  go  out. 

P.  Cover  it  with  something.  May  I?  Do  you  see 
now  the  flame  is  out  again. 

M*  How  can  you  express  that  knowledge? 


Fig.  II. 


64  CONVERSATIONS  ON  CHEMISTRY. 

P.  A  candle  can  only  burn  for  a  short  time  in  a  covered 
glass. 

M.  Must  it  only  be  a  covered  glass? 

P.  I  don't  think  so. 

M.  It  needn't  be  any  one  thing.  An  extinguisher 
puts  a  candle  out,  as  you  know,  and  it  is  made  of  metal. 
But  why  does  a  candle  burn  in  a  lantern? 

P.  Because  it  has  air-holes. 

M.  What  have  they  to  do  with  it? 

P.  Fresh  air  always  comes  in  at  them,  and  the  used-up 
air  gets  out  at  the  top  air-hole. 

M.  Now  just  try  to  put  together  all  that  we  have 
spoken  about. 

P.  The  candle  requires  air  to  burn.  In  a  closed  space 
a  candle  can  burn  only  a  short  time.  If  the  air  in  this 
space  is  changed,  the  candle  can  burn  longer. 

M.  Good.  But  this  room  is  a  closed  space,  and  yet 
the  candle  can  burn  here  as  long  as  it  will  last. 

P.  Yes,  because  the  room  is  so  big. 

M.  There  you  have  discovered  something.  So  you 
think  that  the  larger  a  closed  space  is  the  longer  a  candle 
will  burn  in  it? 

P.  Yes. 

M.  It  is  so.  But  there  are  many  important  conclu- 
sions to  be  drawn  from  it.  Can  you  give  me  any  reason 
why  that  is  the  case? 

P.  No. 

M.  We  will  look  for  resemblances.  A  short  candle 
will  only  burn  a  short  time,  a  long  candle,  a  long  time. 
Why? 

P.  Because  in  burning  the  candle  gets  used  up. 
Should  the  air  get  used  up  by  burning? 

M'  Look    for    a    moment.     I    have    here    a    candle 


COMBUSTION. 


6S 


attached  to  a  wire,  and  lower  it,  burning,  into  a  bottle 
(Fig.  12).      When  it  has  gone  out  I 
take  it    carefully   out    and   light   it. 
If   I   put   it   at   once   in   the   bottle 
again — 

P.  It  goes  out  immediately. 

M.  It  follows  from  that  that  the 
air  in  the  bottle  is  used  up. 

P.  How?  There  is  some  there 
still. 

M.  That  isn't  air.  Air  has  the 
property  that  a  candle  can  burn  in 
it.  What  is  in  the  bottle  has  not  this 
property. 

P.  But  it  looks  just  like  air. 

M.  Quite  so;  what  is  in  there  is 
a  colourless  gas  like  air,  but  not 
what  we  call  air.  A  chemical  change 
has  taken  place  with  the  air,  and  it  has  other  properties. 

P.  Other  properties?  Yes,  the  candle  doesn't  bum 
any  longer.  But  beyond  that  I  don't  see  any  other 
properties. 

M.  That  is  because  nearly  all  gases  look  very  much 
like  each  other.  The  difference  of  their  properties 
is  only  brought  to  light  after  careful  searching.  I  have 
here  a  large  flask  with  ordinary  mortar  shaken  up  with 
water  and  left  for  a  while.  Most  of  the  mortar  has  sunk 
to  the  bottom,  but  a  little  has  dissolved  in  the  water. 
It  seems  also  as  if  the  water  had  kept  its  properties; 
it  doesn't  look  any  different.  But  still  it  has  changed. 
Taste  it! 

P.  How  unpleasant!  lik^  soap!     It  isn't  poisonous? 

M.  No.      I  pour  some  of  the  lime-water  into  a  bott)e 


Fig.  12. 


66  CONyERSATlONS  ON  CHEMISTRY. 

Vfhich  has  ordinary  air  in  it,  and  shake  it.  What  do 
you  see? 

P.  Nothing  much. 

M.  The  Hme-water  remains  unchanged.  Now  I  do 
the  same  with  the  bottle  where  the  candle  burnt. 

P.  The  water  is  becoming  quite  milky. 

M.  So  you  see  that  the  gaseous  contents  of  the  bottle 
in  which  the  candle  burnt  has  a  property  that  ordinary 
air  doesn't  possess.  The  air  then  has  gone  through  a 
chemical  change. 

P.  So  you  can  see  by  means  of  lime-water  what  you 
can't  see  with  your  eyes? 

M.  Yes.  If  we  could  see  the  new  things  in  the  air 
without  help,  we  wouldn't  need  to  make  use  of  lime- 
water.  A  substance,  which  in  such  a  way  makes  known 
something  present,  is  called  a  reagent,  and  the  event 
which  is  called  forth  by  it,  a  reaction. 

P.  A  reaction  means  that  one  thing  acts  on  another  ? 

M,  Yes;  the  changed  air  and  lime-water  work  upon 
each  other,  and  so  the  white  substance  is  formed 
which  makes  it  cloudy.  But  now  we  will  go  still  deeper 
into  the  subject.  What  happens  to  the  candle  by 
burning  ? 

P.  It  vanishes. 

M.  Do  you  mean  it  goes  quite  away? 

P.  Yes.     Nothing  remains  over  from  it. 

M.  But  if  your  book  or  your  apple  goes  away,  then 
you  ask  where  they  can  be.  And  in  the  same  way 
with  everything  else. 

P.  Yes,  they  can't  vanish. 

M.  But  the  candle? 

P.  H'm!  But"  where  can  it  be?  It  really  vanished 
before  my  eyes. 


COMBUSTION.  07 

M.  Yes,  it  became  invisible.  Can't  it  have  changed 
into  something  invisible? 

P.  There  isn't  anything  invisible. 

M.  Oh!  isn't  there? 

P.  No,  there  aren't  any  spirits  or  ghosts. 

M.  It  seems  that  even  they  are  sometimes  visible. 
But  can  you  see  air? 

P.  No.     But  air  is  changed  by  burning.     I  can't 

understand  that. 

M.  It  is  quite  simple.  The  candle  and  air  transform 
each  other  mutually  by  burning,  and  a  gaseous  substance 
is  the  result,  which,  owing  to  this  state,  cannot  be  seen. 

P.  Gaseous  substances  that  aren't  air? 

M.  So  that  is  your  difficulty?  You  know  that  many 
liquids  look  like  water  that  aren't  water.     In  the  same 

way    there    are    many    gases    that  

look  like  air,  but  are  something 
quite  different.  On  that  account 
in  the  earlier  developments  of 
chemistry  there  was  great  diffi- 
culty, till  characteristics  like  that 
with  lime-water  taught  people  to 
distinguish  between  the  different 
gases.  But  now  we  will  try  another 
experiment.  I  light  a  candle  again, 
and  hold  a  large  empty  glass  over 
it  (Fig.  13).     What  do  you  see? 

P.  The  glass  is  becoming  cloudy, 
as  if  it  had  been  breathed  on. 

M.  What  is  the  cloudiness  that  ap- 
pears  on  a  glass  when  breathed  upon  ? 

P.  I  know  that;  it  is  made  by  drops  of  water  which 
come  from  warm  breath,  and  are  laid  on  the  cold  surface. 


68  CONIFERS  A  TIONS  ON  CHEMISTRY. 

M.  Right.  What  appears  in  the  glass  consists  also 
of  drops  of  water. 

P.  How  do  they  get  there? 

M.  The  candle  in  burning  changes  itself  partly  into 
water. 

P.  That  is  extraordinary!  I  never  thought  of  that. 
But  water  won't  make  the  lime-water  cloudy? 

M.  No,  water  never  does  that.  Two  new  substances 
are  formed  when  a  candle  burns.  One  is  water  and 
the  other  is  that  which  makes  lime-water  cloudy. 

P.  What  is  it  called? 

M.  Carbonic  acid  gas. 

P.  That  is  a  funny  name.     What  does  it  mean? 

M.  You  can  find  that  out  later  on. 

P.  The  whole  thing  is  becoming  more  and  more  mud- 
dhng! 

M.  You  are  right.  We  will  examine  simpler  cases 
first;  if  you  understand  them,  you  will  understand 
the  others.     We  are  going  to  burn  iron. 

P.  Can  you? 

M.  Quite  easily.     You  know  what  iron  filings  are? 

P.  Yes,  they  are  little  specks  of  iron  which  have 
fallen  down  in  filing. 

M.  I  sprinkle  some  iron  filings  in  the  flame. 

P.  How  pretty!     Just  like  Httle  stars! 

M.  That  is  burning  iron. 

P.  Why  didn't  the  iron  gauze  burn  when  I  held  it 
in  the  flame? 

M.  It  wasn't  hot  enough,  for  the  heat  is  conducted 
off  by  the  gauze.  But  the  little  iron  specks,  on  the  other 
hand,  are  quickly  heated,  and  lose  none  of  their  heat 
by  being  conducted  off. 

P.  Then  large  pieces  of  iron  ought  to  burn,  if  they  are 
made  hot  enough. 


COMBUSTION. 


69 


M.  And  so  they  do.  Later  on  we  will  burn  iron  gauze 
itself.  Iron  burns  also  on  melting,  if  it  is  glowing.  The' 
burnt  iron  breaks  off  with  hammering. 

P.  But  you  don't  see  any  flame. 

M.  There  is  such  a  thing  as  burning  without  flame. 
The  little  stars  from  the  iron  filings  were  not  flames. 
We  will  make  an  experiment  like  that  now.  This  black 
powder  is  also  iron,  only  in  far  smaller  pieces  than  ordi- 
nary iron  filings.  I  put  a  small  wire  tripod  on  the  bal- 
ance, lay  a  narrow  piece  of  wire  gauze  on  it,  and  shake 
out  several  grams  of  iron  powder  on  it  (Fig.  14). 
The  whole  thing  is  now  made  to  balance.    Then  I  hold 


Fig.  14. 


a  flame  to  the  edge  of  the  little  heap.  Now  it  is  begin- 
ning to  burn. 

P.  I  only  see  it  glowing. 

M,  That  is  how  powdered  iron  burns.  Charcoal  can 
only  glow  when  it  burns. 

P.  That  is  true.  But  why  have  you  put  it  all  on  a 
balance  ? 


70 


CONVERSATIONS  ON  CHEMISTRY. 


M.  You  will  soon  see.  What  do  you  think?  will 
iron  become  lighter  or  heavier  by  burning? 

P.  I  should  think  lighter.     The   scale   with   the   iron 
will  rise. 
M.  Notice  carefully. 

P.  It  is  sinking!     Perhaps  it  was  only  a  draught.    No, 
it  is  getting  heavier.     That  is  very  odd. 
M.  Why? 

P.  One  time  a  thing  becomes  lighter  by  burning, 
another  time  heavier. 

M.  In  the  case  of  the  candle  the  thing  that  was  made 
by  burning  went  away,  with  iron  it  remains.  If  it  stays, 
it  increases  the  weight. 

P.  With  the  candle  as  well?    I'd  like  to  see  that. 
M.  To  do  that,  you  have  only  got  to  keep  hold  of  what  is 
made  by  the  candle  burning — water  and  carbonic  acid  gas. 
P.  That  must  be  rather  difficult. 
M.  Not  very.      There  is  a  sub- 
stance called  caustic  soda  which  has 
the    property  of    binding    together 
every  trace  of  water  and  carbonic 
acid  gas  with  which    it  comes   in 
contact.    I  put  some  loose  pieces  of 
it  in  the  top  part  of  a  lamp-funnel, 
which    I    place    over    a    burning 
candle     (Fig.     15),     and    put    the 
whole  on  the    scale    and    balance 
it.     We  won't   need   to  wait  very 
long. 

P.  Yes,     the     scale     with     the 
candle  is  beginning  to  sink. 
M.  And  the  proportion  will  be  more,  the  more  the 
candle  is  burnt. 


Fig.  15. 


OXYGEN.  71 

P.  Is  it  the  same  with  all  burning  substances? 

M.  Yes ;  you  may  try  to  burn,  after  this,  oil,  petroleum, 
or  matches,  or  whatever  else  you  like,  instead  of  a  candle, 
under  the  cylinder  with  caustic  soda.  The  weight  will 
always  be  increased. 


11.  OXYGEN. 

M.  What  did  you  learn  last  time? 

P.  That  all  bodies  become  heavier  by  burning. 

M.  That  is  not  quite  complete.     Think  of  the  candle. 

P.  That  all  bodies  become  heavier  by  burning  if 
you  take  w^hat  is  formed  into  account. 

M.  Think  of  the  candle  again!  If  it  was  entirely 
burnt  ? 

P.  Ah,  yes!  That  which  is  formed  by  bodies  in 
burning  is  heavier  than  the  body  was  itself. 

M.  Now  it  is  right. 

P.  But  can  iron  burn,  too,  so  that  nothing  remains? 

M.  So  that  no  iron  remains?  Certainly.  Look,  for 
yourself,  what  the  iron  powder  that  we  burnt  yesterday 
has  become. 

P.  It  is  a  dark  mass,  which  looks  rather  like  the 
powdered  iron,  only  it  is  caked  together. 

M.  Take  some,  and  crush  it  in  the  mortar. 

P.  There  is  a  black  powder. 

M.  Now  grind  some  powdered  iron  after  you  have 
cleaned  out  the  mortar. 

P.  It  is  bright  like  iron. 

M.  Now  you  see  the  difference.  The  burnt  iron 
isn't  iron  any  longer,  but  a  substance  with  other  proper- 


72  CONVERSATIONS  ON  CHEMISTRY. 

ties,  and  the  iron  has  vanished  in  the  same  way  that  the 
burnt  candle  vanished. 

P.  But  the  air,  which  helped  with  the  burning? 

M.  The  same  thing  happened  to  it  which  happened 
to  the  iron.  In  the  same  way  as  the  solid  iron  changed 
to  the  solid  substance,  smithy  scales,  the  vanished  part 
of  the  air  used  by  burning  a  candle  becan^e  part  of 
another  gas. 

P.  Has  another  gas  been  made  by  iron  as  well? 

M.  No. 

P.  Then  air  must  vanish  when  iron  burns  in  it. 

M.  We  will  make  the  experiment.  I  put  my  tripod 
with  powdered  iron  on  a  little  floating  board,  hght  it, 
and  cover  it  with  a  large  glass,  so  placed  that  it  stands 
on  the  bottom  (Fig.   i6).     As  the  experiment  is  rather 


slow,  we  must  wait  till  the  glowing  iron  is  extinguished 
and  has  become  cold.     What  do  you  see  now? 

P.  It  really  seems  as  if  air  had  vanished;    but  only  a 
part,  less  than  a  quarter. 


OXYGEN.  73 

M.  If  you  measure  it  more  closely,  it  is  about  a  fifth. 

P.  Perhaps  you  took  too  Httle  iron. 

M.  No.  If  I  had  taken  morC;  no  more  air  would  have 
been  used.    - 

P.  But  this  is  quite  different  from  a  candle,  and  also 
from  iron.     You  can  burn  them  completely. 

M.  Can  you  bum  wood  entirely?. 

P.  Ash  remains. 

M.  It  is  the  same  with  air.  Wood  is  a  mixture  of 
combustible  and  non-combustible  substances;  when  the 
former  are  burnt,  the  latter  remain  behind.  Air  is  a 
mixture  of  two  gases ;  the  one  gets  separated  by  burning, 
and  is  called  oxygen,  the  other  is  left  unchanged,  and  is 
called  nitrogen.  Oxygen  only  takes  up  about  a  fifth 
part  of  the  space  of  air. 

P.  If  you  had  pure  oxygen,  would  it  entirely  vanish 
in  burning? 

M.  Certainly,  if  there  were  no  other  gas.  We  will 
make  pure  oxygen. 

P.  Can  we? 

M.  It  has  been  possible  for  rather  more  than  a  hundred 
years.  This  white  salt  is  called  potassium  chlorate. 
When  it  is  heated  a  great  deal  of  oxygen  is  formed. 

P.  What  sort  of  brown  powder  are  you  mixing  with  it  ? 

M.  That  is  heated  iron-rust.  When  some  is  put  with 
it,  the  oxygen  forms  more  easily  and  regularly.  I 
shake  the  mixture  in  a  Httle  round  flask.  Now  I  must 
make  my  apparatus.  To  do  that,  I  take  a  cork  which 
just  fits  into  the  neck  of  the  flask,  and  cut  a  piece  of 
glass  tubing. 

P.  How  can  you  cut  glass? 

M.  It  isn't  exactly  cut,  but  broken.  But  so  that  the 
fracture  comes  at  the  right  place,  and  is  even  all  round, 


74 


CONVERSATIONS  ON  CHEMISTRY. 


I  must  scratch,  and  divide  the  tube  at  the  place  I 
wish. 

P.  What  sort  of  tool  is  this? 

M.  This  is  an  old  three-cornered  file  whose  teeth 
are  ground  down  so  that  three  cutting  edges  are  made. 
If  I  saw  the  glass  tube  with  this  sharp  edge,  it  will  crack. 
Then  I  break  the  tube  apart  with  the  crack  furthest 
from  me,  so  that  it  breaks  off  evenly  (Fig.  17). 


Fig.  17. 


P.  That  is  clever!     Can  I  do  it  too? 

M.  I  will  give  you  a  piece  of  glass  tube  afterwards 
and  you  can  practice  on  it.    Now  I  will  bend  the  tube. 

P.  You  can't;    it  will  break. 

M.  Heat  will  make  glass  soft,  so  that  it  can  be  bent. 
I  put  the  part  where  the  bend  is  to  come  in  the  flame 
and  turn  the  tube  continually,  so  that  it  shall  be  evenly 
heated,  otherwise  it  would  crack.  After  a  time  the 
glass  will  become  so  soft  that  it  will  bend  with  its  own 
weight.  I  help  it  a  little  till  it  is  the  right  shape,  and  let 
the  glass  get  cold  and  hard,  then  it  will  keep  its  new  shape. 

P.  That  looks  so  easy.     Can  I  do  it  too? 

M.  It  isn't  difficult,  but  it  needs  practice.  The  main 
thing  is,  not  to  apply  the  heat  to  any  one  point,  and 
only  to  use  very  light  pressure  in  bending  it.     Otherwise 


OXYGEN.  .  75 

the  bend  may  easily  come  uneven.  Now  the  other  end 
of  the  tube  must  be  bent  a  Httle,  and  finally  I  turn  each 
end  in  the  flame  for  a  little,  so  that  the  sharp  edges  become 
round  and  can't  cut  or  scratch  any  more.  That  must 
never  be  forgotten. 

P.  How  is  that  managed? 

M.  Soft  glass  behaves  like  liquids.  You  know  that 
instead  of  having  corners  or  points,  their  surfaces  are 
always  rounded. 

P.  Why  do  liquids  do  that? 

M.  On  account  of  the  surface  tension.  Through  this 
the  surface  endeavours  to  become  as  small  as  possible, 
and  as  a  ball  is  the  form  which  contains  the  most  contents 
with  the  least  surface,  all  liquids  try  to  form  themselves 
into  the  shape  of  a  ball. 

P.  But  liquids  take  the  form  of  the  vessel  that  contains 
Inem. 

M.  Quite  right.  This  comes  from  gravity,  as  they 
always  try  to  get  as  low  down  as  possible.  Both  causes 
work  simultaneously  on  the  liquid,  but  the  gravity  is 
generally  far  the  stronger,  and  the  shape  of  the  water  is 
most  dependent  on  that.  Now  we  must  make  a  hole  in 
the  cork.  For  that  I  bore  a  hole  with  a  steel  point,  an 
awl,  and  then  I  file  it  with  a  somewhat  large  round  file  till 
I  can  with  a  little  force  stick  the  glass  tube  through. 
Now  everything  is  put  together,  and  I  fasten  the 
apparatus,  so  that  I  can  slip  a  piece  of  wire  gauze  below 
the  flask  and  put  the  lamp  under  it  (Fig.  i8). 

P.  Why  do  you  put  the  end  of  the  tube  in  a  dish  of 
water  ? 

M.  To  collect  the  gas.  If  I  were  to  stick  the  tube  in 
an  empty  flask,  that  is,  one  filled  with  air,  and  were  tc 
lead  the  gas  into  it,  it  would  mix  with  the  air,  and  I 


76 


COhiyERSATlONS  ON  CHEMISTRY. 


shouldn't  be  able  to  see  when  the  flask  was  full.  So 
I  fill  the  flask  with  water,  and  allow  it  to  be  displaced  by 
the  entering  gas,  holding  the  mouth  of  the  flask  over 


Fig.  i8. 


the  glass  tube.  As  the  gas  doesn't  mix  with  water,  I 
get  it  pure. 

P.  Bubbles  of  gas  are  rising;  hold  the  flask  over  them! 

M.  At  first  it  is  only  the  air  which  was  in  the  appa- 
ratus. 

P.  Then  how  do  you  know  when  the  new  gas  comes? 

M.  I  take  the  tube  out  of  the  water  and  hold  a  glowing 
splinter  at  its  open  end.     What  do  you  see  ? 

P.  It  goes  on  glowing. 

M.  Then  it  is  only  air.     And  now? 

P.  Oh,  it  begins  to  burn  of  itself! 

M.  Not  of  itself,  but  with  the  oxygen  which  comes 
out.  Now  I  bring  the  tube  below  the  water  again,  and 
hold  the  flask  over  it.  But  in  order  not  always  to  have 
to  hold  it,  I  place  it  in  a  little  lead  stand,  below  which 
the  tube  ends,  so  that  the  bubbles  rise  in  the  flask 
and  expel  the  water  (Fig.  19).     In  the  mean  time  I  will 


OXYGEN. 


77 


fill  some  flasks  with  water  so  that  we  may  afterwards  fill 
them  with  oxygen. 

P.  Please  show  me  the  ex- 
periment with  the  glowing  splin- 
ter again. 

M.  It  is  the  test  for  oxygen. 
Whenever  a  glowing  spHnter  is 
put  into  oxygen  it  catches  fire. 
I  can  repeat  the  experiment 
often  with  the  oxygen  in  this 
flask.  But  at  last  it  gets  used 
up,  and  the  experiment  won't 
succeed  any  more. 

P.  What  is  the  reason  of 
that?  ^iG-  ^9- 

M.  I  will  first  show  you  some  other  similar  experiments. 
I  tie  a  piece  of  charcoal  on  to  a  wire,  light  it  at  one  corner, 
and  place  it  in  the  oxygen.  It  soon 
begins  to  glow  all  over,  much  more 
brightly  than  in  air.  A  bit  of  sul- 
phur in  a  little  iron  spoon  which 
you  can  hardly  see  burning  in  the 
air  gives  a  bright  blue  flame.  A 
piece  of  phosphorus  in  a  similar 
spoon,  which  burns  in  the  air  with 
a  yellow  flame,  looks  as  bright  as 
the  sun.  A  thin  spiral  of  iron  wire, 
on  the  end  of  which  a  little  tinder 
has  been  fastened  and  made  to 
glow,  catches  fire  and  burns, 
throwing  out  sparks,  and  the 
smithy  scales  fall,  while  hot,  into  the  water  which  covers 
the  bottom  of  the  flask.    It  is  better  to  put  a  little  sand 


Fig.  20. 


78  CONVERS/iTIONS   ON  CHEMISTRY. 

at  the  bottom  of  the  flask  so  as  to  keep  it  from  breaking 
(Fig.  20). 

P.  Oh,  that  is  a  beautiful  firework! 

M.  We  won't  forget  what  the  firework  means.  What 
can  you  say  in  general  about  these  experiments? 

P.  Things  burn  much  more  brightly  in  oxygen  than 
in  air. 

M.  Quite  right.  But  they  burn  in  air  too,  at  the 
expense  of  the  oxygen  that  is  present.  Where  does  the 
difference  lie? 

P.  They  give  off  more  heat  when  in  pure  oxygen. 

M.  Your  answer  is  right  or  wrong  according  to  what 
you  mean  by  the  word  heat.  If  you  mean  to  say  that 
the  quantity  of  heat  that  i  gram  of  carbon  or  iron 
gives  out  is  greater  when  it  burns  in  oxygen  than  when 
it  burns  in  air,  that  is  wrong.  The  quantity  of  heat 
is  the  same.  But  if  you  mean  that  the  temperature  rises 
higher,  that  is  right. 

P.  I  mean  the  temperature. 

M.  Of  course  you  do!  This  is  the  reason.  The  same 
amount  of  heat  which  is  given  out  in  both  cases  has 
only  to  heat  the  resulting  product  of  combustion  when 
the  substance  burns  in  pure  oxygen;  but  when  it  burns 
in  air  it  has  to  heat  the  nitrogen  which  is  mixed  with 
the  oxygen. 

P.  Is  the  brighter  light  connected  with  the  higher 
temperature  ? 

M.  Certainly.  The  temperature  can  even  be  estimated 
from  the  brightness  of  the  light.  But,  besides  that,  the 
higher  temperature  makes  the  burning  take  place  more 
quickly. 

P.  What  has  that  got  to  do  with  it  ? 

M'  It  has  been  generally  found  that  chemical  changes 


OXYGEN.  79 

take  place  more  quickly  the  higher  the  temperature. 
But  we  will  go  back  to  oxygen  again.  The  phenomena 
you  have  seen  are  all  chemical  changes,  for  the  burning 
substances  and  the  oxygen  have  disappeared,  and  new 
substances  have  been  produced  instead. 

P.  Are  the  heat  and  the  light  which  have  been  pro- 
duced, new  substances  too? 

M.  No,  these  things  are  not  called  substances,  because 
they  possess  neither  weight  nor  mass. 

P.  But  all  the  same  they  are  really  there. 

M.  Certainly,  because  they  are  real  things.  They 
behave  something  like  substances,  for  they  change  into 
one  another,  and  new  quantities  of  them  can  never  be 
produced  except  by  such  change.  Only  they  have  no 
weight  like  substances. 

P.  Then  these  must  be  what  are  called  forces? 

M.  People  used  to  call  these  things  forces,  but  that 
led  to  a  misunderstanding,  since  the  word  force  had 
already  been  used  for  something  different.  Now  they 
are  called  energies.  Heat  is  one  kind  of  energy,  and 
light  another. 

P.  Yes;  people  are  said  to  be  energetic  who  can  do 
something  and  carry  it  through. 

M.  The  scientific  use  of  the  word  energy  is  pretty 
nearly  the  same.  Energy  is  what  causes  things  to 
change. 

P.  So  when  substances  change  by  chemical  action, 
is  that  energy  too? 

M.  To  be  sure;  only  we  express  it  somewhat  differ- 
ently. We  say  that  substances  possess  chemical  energy 
when  they  are  able  to  act  on  each  other  and  produce 
new  substances.  At  the  same  moment  that  the  substances 
change,   a  change  of  a  part  of  their  chemical  energy 


8o  CONVERSATIONS   ON  CHEMISTRY. 

takes  place,  and  this  assumes  the  form  of  heat  or  light, 
and  sometimes  of  electrical  or  mechanical  energy. 

P.  That  strikes  me  as  very  curious  and  mysterious. 

M.  The  change  of  one  kind  of  energy  into  another  is 
not  more  mysterious  than  the  change  of  one  substance 
into  another;  indeed,  it  is  even  simpler.  To  tell  you  a 
little  more  about  energy :  you  must  know  that  the  ordinary 
work  which  a  man  or  a  horse  or  a  steam-engine  does  is 
also  energy. 

P.  Then  I  ought  to  be  able  to  make  heat  or  Hght  or 
electricity  by  my  arms. 

M.  So  you  can;  when  you  rub  your  hands  together 
they  grow  warm,  and  if  you  turn  a  blunt  drill  with  all 
your  force  in  a  hole,  it  soon  becomes  so  hot  that  you 
could  burn  your  finger  with  it.  And  you  know  already 
that  people  can  make  fire  by  friction. 

P.  Yes,  that  is  true.  So  I  can  make  as  much  heat 
as  I  like? 

M.  Not  as  much  as  you  like,  but  as  much  as  you  can. 
When  you  turn  the  drill  for  some  time  you  can't  go  on 
any  more;  you  are  quite  exhausted;  that  is,  you  have 
used  up  the  store  of  energy  which  you  possessed. 

P.  Where  did  I  get  that  energy  from? 

M.  From  your  food.  It  is  chemical  energy  which 
you  have  taken  in  with  your  food,  and  in  your  body 
there  is  a  kind  of  apparatus,  the  muscles,  which  change 
chemical  energy  into  work. 

P.  How  do  they  do  that? 

M.  I  wish  I  knew!  Investigators  haven't  found  out 
yet  how  it  is  done.  But  there  is  no  doubt  that  chemical 
energy  is  used  up  in  doing  work,  for  you  see  that  a  hard 
worked  horse  must  be  well  fed  in  order  to  do  its 
work. 


OXYGEN.  8i 

P.  But  I  always  have  a  good  appetite,  even  when  I 
do  nothing. 

M.  Then  you  waste  the  chemical  energy  of  your  food. 
Of  course  you  always  want  a  certain  quantity  in 
order  to  keep  your  temperature  up  to  37°  C;  for  as 
your  body  is  considerably  warmer  than  its  surround- 
ings, it  is  continually  losing  heat  which  must  be  replaced 
by  means  of  food.  That  is  a  second  way  in  which  you  can 
make  heat,  although  it  is  beyond  your  control. 

P.  Can  I  make  light  too? 

M.  Yes;  if  you  rub  two  pieces  of  sugar  together  in 
the  dark,  they  will  make  light. 

P.  Don't  they  make  light  in  the  daytime? 

ikf.  Yes;  only  the  Hght  is  so  weak  that  it  can't  be  seen 
by  daylight.  This  experiment  shows  that  the  work  of 
your  muscles  can  be  changed  into  light. 

P.  But  can't  I  make  light  without  anything? 

M.  You  can't;  but  glow-worms  and  little  animals, 
which  are  the  cause  of  the  phosphorescence  of  the  sea, 
can.  These  change  the  chemical  energy  of  their  food 
directly  into  light. 

P.  And  can  I  make  elec^trical  energy  too? 

M.  Certainly ;  you  have  only  to  rub  a  piece  of  sealing- 
wax  with  a  cloth. 

P.  Oh,  yes,  I  know.  But  I  must  use  my  muscles  again 
to  do  that ;  I  don't  do  it  directly. 

M.  Electric  currents  run  through  your  body  when- 
ever you  exert  yourself,  indeed  whenever  you  think. 
But  they  stay  in  your  body  and  it  isn't  easy  to  conduct 
them  outside. 

P.  I  never  dreamt  that  I  could  do  all  these  things! 

M.  Well,  you  needn't  be  conceited  about  it,  for  every 
animal  can  do  the  same. 


S2  CONyERSATlONS   ON  CHEMISTRY. 

P.  Still,  it's  very  queer.  Where  does  the  energy  of 
food  come  from? 

M.  From  the  sun. 

P.  I  don't  understand  that. 

M.  Where  does  food  come  from?  Either  from  plants 
or  animals.  Plants  grow  only  in  sunlight,  for  they  use 
the  energy  of  hght  to  build  up  their  structures;  they 
store  it  up  in  this  form.  And  we  consume  the  energy 
of  the  sun  in  the  plants.  And  the  animals  whose  flesh 
we  eat  subsist  upon  plants,  that  is,  upon  the  energy  of  the 
sun. 

P.  I  shall  think  of  the  sun  quite  differently  after 
this. 

M.  If  you  only  think  of  what  we  have  been  speaking 
about,  you  will  understand  more  of  the  world  than  you 
did. 


12.  COMPOUNDS  AND  CONSTITUENTS. 

M.  Last  time  you  learned  a  great  deal  that  was  new 
to  you.     Tell  me  the  principal  things. 

P.  First  I  learned  how  oxygen  is  made  and  collected; 
then  I  learned  that  substances  burn  far  more  brightly  in 
it  than  in  ordinary  air,  and  that  is  because  air  contains 
only  a  fifth  part  of  oxygen.  Then  I  learned  something 
about  energy.  But  that  was  so  much  and  so  unfamiliar 
that  I  can't  say  it  in  a  few  words. 

M.  We  will  try  together.  Wherein  does  energy  resem- 
ble substances,  and  wherein  does  it  differ  from  them? 

P.  Resemble?  Yes,  it  can  change  into  many  kinds, 
and  when  one  kind  is  formed,  others  vanish. 

M.  Right.     Where  do  they  differ? 


COMPOUNDS  AND  CONSTITUENTS.  ^Z 

P,  Energy  can't  be  weighed,  and  it  comes  from  the 
sun  on  to  the  earth.     Substances  don't  come  from  there. 

M.  No,  at  any  rate  not  in  detectable  quantity. 
Now,  first  of  all,  be  sure  of  these  points;  the  others  will 
be  much  more  comprehensible  if  we  are  careful  over 
these.  Now  we  will  return  to  oxygen.  There  is  still 
here  a  flask  which  was  filled  yesterday.  What  noticeable 
properties  has  it? 

P.  Oxygen  looks  like  air;  it  is  colourless. 

M.  What  does  it  smell  like? 

P.  I  can't  smell  anything;  it  has  no  smell. 

M.  You  should  have  been  able  to  tell  me  that  without 
opening  the  bottle.  Just  think,  a  fifth  part  of  the  air  is 
made  of  oxygen. 

P.  Oh,  yes,  because  air  doesn't  smell,  oxygen  can't. 

M.  These  are  the  noticeable  properties  of  oxygen. 
Besides  these  it  has  others,  which  can  only  be  found  out 
by  measurements  or  experiments.  The  burning  phenom- 
ena that  I  showed  you  are  also  such  properties.  They 
are  called  chemical  properties,  because  they  depend  upon 
chemical  processes.  Also  the  reaction  of  oxygen,  the 
bursting  into  flame  of  a  piece  of  glowing  wood,  is  one 
of  these  chemical  properties.  Now  we  will  learn  another 
way  of  telling  oxygen.  This  brick-coloured  powder  is 
called  mercuric  oxide.  I  put  some  into  a  test-tube  made 
out  of  a  particular  kind  of  glass  that  has  rather  thicker 
sides,  and  is  more  difficult  to  melt  than  ordinary  glass, 
and  attach  a  gas-delivery  tube  as  before.  Then  I  make 
the  glass  hot  with  a  lamp.     What  do  you  see? 

P.  The  red  powder  is  becoming  black.  It  is  becom- 
ing charred. 

M.  No;   if  I  let  it  get  cold  it  will  become  red  again. 

P.  Then  how  does  it  get  black? 


84 


CONVERSATIONS   ON  CHEMISTRY. 


M.  There  are  many  substances  which  change  their 
colour  with  heating.  Colour  depends  to  a  great  extent 
on  temperature. 

P.  Now  there  are  bubbles  coming  (Fig.  21). 


Fig.  21. 


M.  Again  it  is,  first  of  all,  air  which  has  expanded  by 
heat. 

P.  But  now  the  bubbles  are  much  more  frequent  and 
continuous. 

M.  We  will  collect  some  of  the  gas  in  a  little  test-tube 
and  try  it  with  the  glowing  splinter.  It  is  still  air  that 
is  coming  out  of  the  test-tube.  But  the  second  time  it  is 
filled— 

P.  The  splinter  has  caught  fire;  it  is  oxygen. 

M.  Perhaps.  We  will  collect  some  and  see  if  it  is 
colourless  and  scentless.     Try  it! 


COMPOUNDS  AND  CONSTITUENTS.  85 

F.  Yes,  it  is  scentless,  and  one  can't  see  any  colour. 
But  why  was  it  necessary  to  prove  it  in  this  way  as  well  ? 

M.  Before  one  can  say  that  anything  that  one  has  is 
a  definite  substance,  one  must  have  made  sure  that 
all  its  properties  are  the  right  ones. 

P.  But  then,  one  can't  look  for  all  its  properties;  there 
would  be  no  end  to  it. 

M.  There  you  are  right.  But  several  properties  must 
always  be  tested  for,  because  it  often  happens  that  dif- 
ferent substances  have  one  common  property,  whereas 
other  properties  are  different. 

P.  Really  just  the  same  property? 

M.  One  can't  be  absolutely  certain,  even  when  no 
difference  can  be  seen.  Since  no  property  can  be  noticed, 
or  measured  with  absolute  exactness,  one  can't  be  certain 
that  a  seeming  resemblance  will  not  turn  out  to  be  a 
difference  on  closer  inspection.  But  just  to  make  these 
difficult  researches  unnecessary,  people  examine  several 
properties.  For  it  is  very  seldom  that  two  different 
substances  have  several  properties  in  common. 

P.  Look  what  has  happened  to  the  experiment  mean- 
while.     The  test-tube  looks  just  like  silver  at  the  top. 

M.  Yes,  and  the  greater  part  of  the  mercuric  oxide 
has  disappeared.  I  heat  it  a  little  longer  and  now 
it  is  all  gone.  I  take  the  delivery-tube  out  of  the  water 
and  let  everything  cool. 

P.  Why  don't  you  leave  it  all  as  it  is? 

M.  The  hot  oxygen  gas  in  the  tube  in  cooling  would 
contract,  and  the  water  might  enter  the  test-tube.  Now 
look  closely:  the  silvery  stuff  in  the  tube  can  be  brushed 
together  with  a  feather,  and  changes  into  bright  liquid 
droplets. 

P.  They  look  just  like  mercury. 


86  CONVERSATIONS  ON  CHEMISTRY. 

M.  They  are  mercury. 

P.  But  how  did  it  come  there? 

M.  It  came  out  of  the  mercuric  oxide. 

P,  And  has  the  oxygen  been  made  from  it  too? 

M.  Yes,  these  two  substances,  and  nothing  else. 

P.  But  why  isn't  the  mercury  where  the  mercuric  oxide 
was? 

M.  Because  the  mercury  with  the  heat  from  the  lamp 
became  volatile,  that  is,  it  changed  to  a  vapour.  Then, 
when  the  tube  was  colder,  the  vapour  changed  back 
again  to  liquid  mercury.  I  will  now  take  some  more 
mercury  in  a  test-tube,  and  heat  it;  look,  the  first  drops 
are  forming,  it  is  becoming  thicker,  and  now  it  looks  like 
a  silver  looking-glass.  I  repeat  the  experiment  with 
the  liquid  metal  that  I  made  before;  you  see  it  behaves 
just  the  same;  it  is  mercury  too.  But  take  care  of  the 
vapour;   it  is  poisonous. 

P.  I  shouldn't  have  thought  so! 

M.  Why  not? 

P.  Mercury  is  a  metal,  and  metals  don't  boil. 

M.  Certainly  they  do,  only  the  boiling-point  of  most 
of  the  best-known  metals  lies  so  high  that  it  can't  be 
reached  by  ordinary  means.  But,  for  example,  in  the 
flame  of  the  electric  arc  all  known  metals  turn  to 
vapour.  Mercury,  however,  boils  fairly  easily  at  350°  C. 
But  now  we  will  go  back  to  our  experiment.  You  saw 
that  by  heating,  the  red  powder  changed  into  mercury 
and  oxygen.  Out  of  mercury  and  oxygen  red  mercuric 
oxide  can  be  made  again.  You  can,  so  to  speak,  reverse 
the  reaction. 

P.  That  is  wonderful.     Can  I  see  it? 

M.  Unfortunately  I  can't  show  you.  Mercury  oxide 
is  fonned  out  of  mercury  and  oxygen,  if  they  are  left  in 


COMPOUNDS  AND  CONSTITUENTS,  ^7 

contact  together  at  a  temperature  something  over 
300°.  But  that  takes  place  so  slowly  that  it  would 
take  a  week  to  get  a  couple  of  grams.  But  if  it  is 
done  it  shows  exactly  the  same  properties  as  mercury 
oxide. 

P.  Isn't  it  made  in  that  way,  then  ? 

M.  No,  it  is  made  in  quite  a  different  way,  which  you 
wouldn't  understand  yet. 

P.  Then  it  doesn't  matter  which  way  it  is  made? 

M.  Certainly;  there  is  an  important  general  law,  that 
a  definite  substance  always  has  the  same  properties  in 
whatever  way  it  may  have  been  made. 

P.  I  shouldn't  have  thought  so! 

M.  You  have  just  had  an  example  of  it:  the  oxygen 
made  from  mercuric  oxide  had  exactly  the  same  properties 
as  that  made  from  potassium  chlorate. 

P.  Yes,  so  it  did.  I  never  thought  of  that;  I  took  it 
for  granted. 

M.  You  see  again:  People  take  things  for  granted 
when  they  don't  think  about  them.  Now  notice  some 
new  names;  because  from  one  single  substance,  mer- 
curic oxide,  two  different  substances,  mercury  and 
oxygen,  can  be  made,  and  vice  versa;  from  the  two 
latter,  again,  a  single  substance,  mercuric  oxide,  can  be 
made;  the  latter  is  called  a  compound  and  the  former 
the  constituents.     So  mercuric  oxide  is — ? 

P.  Mercuric  oxide  is  a  compound  of  mercury  and 
oxygen. 

M.  Yes,  and  mercury  and  oxygen  are  the  constituents 
of  mercuric  oxide. — Now  we  are  coming  to  an  important 
question  about  the  proportions  by  weight  in  chemical 
processes.  In  this  closed  flask  there  is  oxygen,  and  there 
is  a  piece  of  charcoal  hanging  from  a  wire  in  it.     I  weigh 


88  CON  VERS ATIOhlS  ON  CHEMISTRY, 

the  flask  on  the  balance.  Now  I  will  light  the  charcoal 
without  opening  the  flask. 

P.  How  will  you  do  that? 

M.  I  could  do  it  in  several  ways.  If  I  had  put  a 
second  wire  through  the  stopper,  and  bound  both  wires 
together  with  a  thin  piece  of  iron  wire,  I  could  make  it 
glow  with  an  electric  current,  and  it  would  light  the 
charcoal.  But  since  we  have  sunshine,  I  can  do  it  in  a 
far  simpler  way :  I  shall  light  the  charcoal  with  a  burning 
glass. 

P.  Good;  that's  splendid.  Hurrah!  the  charcoal  is 
burning  already. 

M.  And  now  it  has  gone  out  again,  since  the  oxygen  is 
used  up.  Now  what  do  you  think;  will  the  flask  have 
become  heavier? 

P.  Of  course. 

M.  You  have  taken  it  for  granted  again!  But  we 
will  look.     What  do  you  see? 

P.  The  pointer  is  going  backwards  and  forwards  over 
the  middle.  The  weight  seems  to  have  remained  the  same. 
Perhaps  the  increase  is  so  little  that  it  can't  be  noticed? 

M.  No,  even  with  the  most  careful  weighing  it  would 
always  be  the  same. 

P.  But  that  can't  be  right!  I  learned  and  saw  that 
weight  increased  with  burning. 

M.  The  weight  of  what? 

P.  Ah!  so  it  was.  The  product  of  combustion 
weighed  more  than  the  burnt  body  weighed. 

M.  Well,  and  here? 

P.  Here  it  weighs  the  same. 

M.  That  is  a  false  conclusion.     It  really  weighs  more. 

P.  But  then,  how  is  it  that  the  weight  didn't  change? 

M.  It  is  because  the  oxyen  disappeared.     The  product 


COMPOUNDS  AND  CONSTITUENTS.  89 

of  the  burning  weighs  just  as  much  more  as  the  used 
oxygen.     So  the  gain  and  loss  have  balanced  each  other. 

P.  That  is  extraordinary. 

M,  Yes,  it  is  an  example  of  one  of  the  most  important 
laws,  which  holds  for  all  chemical,  and  also  for  all  physi- 
cal, processes;  whatever  changes  take  place  between  defi- 
nite substances,  they  never  change  their  combined  weight, 

P.  But  the  separate  weights  change? 

M.  Certainly;  but  what  the  one  side  loses,  the  other 
gains.     The  law  only  refers  to  the  sum  of  all  the  weights. 

P.  You  always  taught  me,  in  cases  like  this,  never  to 
ask  why  it  is  so,  but  with  what  it  is  connected.  Is  any- 
thing known  about  it? 

M.  Certainly.  You  know  that  weight  and  mass  are 
in  every  place  proportional.  So  also  the  law  of  the 
unchangeableness  or  conservation  of  mass  holds. 

P.  What  is  the  use  of  this  law? 

M.  It  makes  it  possible  to  account  for  proportions  by 
weight  in  chemical  changes  even  when  you  cannot,  or 
do  not  want  to,  weigh  each  substance  separately.  For 
example,  if  I  weigh  the  amount  of  mercuric  oxide  I 
take,  and  the  amount  of  mercury  I  get  from  it,  then  I 
know  how  much  oxygen  was  there  too.  Because  there 
must  always  be  this  equation:  Mercuric  oxide  =  mer- 
cury +  oxygen,  where  the  name  of  the  stuff  denotes  its 
amount  by  weight. 

P.  Has  oxygen  a  weight?     It  is  a  gas! 

M.  Do  you  think  that  gases  have  no  weight? 

P.  I  can't  believe  it. 

M,  The  density,  or  the  relation  of  the  weight  to  the 
volume,  is  small  with  gases,  several  hundred  times  smaller 
than  with  water.  But  they  certainly  have  weight.  One 
litre  of  ordinary  air  weighs  more  than  one  gram. 


90 


CONVERSATIONS  ON  CHEMISTRY. 


P.  I'd  like  to  see  that. 

M,  I  can  show  you  quite  easily.  Here  is  a  flask  of 
strong  glass  which  I  close  up  with  a  stopper,  in  which 
there  is  a  glass  stop-cock.  So  that  it  shall  not  get  pulled 
out,  I  tie  it  firmly  down  with  wire  or  string.  Now  I 
shall  weigh  it  all.  I  can  pump  air  into  the  flask  through 
the  open  stop-cock  with  a  bicycle  pump.  After  pump- 
ing twice,  I  close  the  stop-cock,  put  the  flask  again  on 
the  balance,  and  it  has  become  distinctly  heavier. 

P.  Can  you  see  how  much  air  you  have  pumped  in? 

M.  Yes;  with  the  aid  of  a  Httle  rubber  tubing  I  con- 
nect the  dcHvery  tube  from  the  oxygen  apparatus  with 
the  stop- cock,  put  a  flask  filled  with  water  over  the  end, 
and  now,  if  I  open  the  stop-cock,  the  air  which  I  pumped 
in  will  come  out,  and  collect  in  the  flask  (Fig.  22).   If  you 


Fig.  22. 


had  weighed  the  flask  exactly  before,  and  weigh  it  again 
now,  the  loss  of  weight  is  the  same  as  the  air  that  has  just 
come  out.  And  if  you  know  the  capacity  of  the  flask, 
you  can  measure  the  volume  of  the  air. 


COMPOUNDS  AND   CONSTITUENTS.  91 

P.  Yes,  so  you  can! 

M.  After  this  you  may  try  to  measure  like  this;  you 
will  find  that  air  is  about  800  times  lighter  than  water. 
Now  we  will  go  back  to  our  experiment.  Have  you 
noticed  the  amount  of  oxygen  I  got  from  potassium 
chlorate  and  mercuric  oxide? 

P.  Yes.  There  seemed  to  be  far  less  from  mercuric 
oxide. 

M.  Yes.  One  gram  of  potassium  chlorate  gives  far 
more  oxygen  than  one  gram  of  mercuric  oxide.  But  if  I 
make  the  experiment  twice,  each  time  with  one  gram  of 
mercuric  oxide,  what  will  be  the  result? 

P.  Each  time  it  will  be  the  same. 

M.  And  with  potassium  chlorate? 

P.  The  same. 

M.  You  think,  then,  that  if  a  substance  is  changed 
into  another,  this  always  happens  according  to  definite 
proportions  by  weight. 

P.  I  don't  know  whether  it  is  quite  definite,  but  it 
must  be  so,  more  or  less. 

M.  It  is  exactly  so.  You  could  have  thought  of  that 
before.  For  a  definite  substance  has  always  quite  defi- 
nite properties;  its  capacity,  in  certain  cases,  to  change 
into  another  substance,  is  one  of  these  properties;  it 
follows  that  the  ratio  of  the  weights  of  the  original  stuff 
and  of  the  product  of  change  must  be  definite. 

P.  I  should  never  have  had  the  courage  to  draw  such 
a  conclusion. 

M.  How  can  it  be  proved  that  such  a  conclusion  is 
right? 

P.  By  experiment. 

M.  Right.  Now  experience  has  shown  for  several 
hundred  years  that  between  the  substances  which  take 


92  CONyERSATIONS  ON  CHEMISTRY. 

part  in  any  change,  roughly  speaking,  a  definite  ratio 
must  exist;  from  a  pound  of  fat  an  unhmited  amount 
of  soap  cannot  be  made,  but  somewhere  about  the  same, 
and  so  on.  But  it  is  only  in  the  last  one  hundred 
years  that  this  question  has  been  carefully  tested  and 
the  law  found  to  be  quite  exact. 

P.  Does  it  apply  to  all  substances? 

M.  To  all  pure  substances;  that  is,  to  those  that 
are  neither  solutions  nor  mixtures. 

P.  It  is  strange.  The  laws  which  you  have  taught 
me  up  till  now  are  all  very  simple  and  easy  to  under- 
stand. But  I'm  afraid  I  will  never  be  able  to  under- 
stand and  use  them  at  the  right  time. 

M.  That  is  only  natural.  A  law  is  like  a  tool:  if  you 
have  had  no  practice,  it  is  of  very  Httle  use  having  the 
tool,  even  if  you  know  what  it  is  for.  But  what  we  are 
going  to  talk  about  later  on  will  give  you  the  necessary 
practice. 


13.  ELEMENTS. 

M.  Last  time  you  learned  two  important  laws,  which 
.show  the  relation  of  the  proportion  by  weight  of  such 
substances  between  which  chemical  changes  take  place. 
The  one  was  called  the  law  about  the  conservation  of 
weight;  just  say  it  over! 

P.  If  chemical  changes  take  place  between  given  sub- 
stances, the  combined  weight  is  not  changed  by  it. 

M.  And  what  is  the  other  law  about? 

P.  About  the  proportion  of  weight  in  chemical  changes. 
If  one  substance  changes  into  another,  the  weight  of  the 
one  has  a  definite  ratio  to  the  weight  of  the  other. 


ELEMENTS.  93 

M.  Right.    It  is  called  the  law  of  constant  proportion. 

P.  But  what  connects  these  ratios? 

M.  That  is  a  sensible  question !  I  can  give  you  a  very 
wonderful  answer  for  it.  But  to  do  that  I  must  first 
make  a  new  idea  clear  to  you :  that  of  chemical  elements. 
You  remember  the  equation:  Mercuric  oxide  =  mercury + 
oxygen;   what  sort  of  quantity  was  -concerned? 

P.  That  of  weight. 

M.  Now,  if  you  split  up  a  definite  quantity  of  mercuric 
oxide  by  heat,  and  collect  the  mercury,  will  it  weigh 
more  or  less  than  the  mercuric  oxide? 

P.  Let  me  think  a  minute.     It  must  weigh  less. 

M.  Why? 

P.  Because  it  weighs  as  much  as  the  mercuric  oxide, 
with  the  oxygen,  and  oxygen  has  weight  too. 

M.  Right.  Then  if  mercury  is  made  into  mercuric 
oxide,  or  oxygen  into  mercuric  oxide,  in  each  case  weight 
is  added:  in  the  one  case,  the  needed  amount  of  oxygen, 
in  the  other. case,  of  mercury. 

P.  I  understand  that. 

M.  You  remember  also  that  we  called  oxygen  and 
mercury  the  constituents  of  mercuric  oxide,  and  the 
latter  a  compound  of  the  former. 

P.  Yes. 

M.  Then  it  follows  that  a  constituent  must  always 
weigh  less  than  any  of  its  compounds. 

P.  Because  something  is  added  each  time.     • 

M.  Quite  right.  Now  you  can  believe  that  all  sorts 
of  chemical  experiments  have  been  made  with  oxygen, 
like  the  one  you  have  seen,  and  that  every  time  the  weight 
of  the  new  substance,  which  was  the  result  of  the  con- 
sumption of  the  oxygen,  was  determined.  No  single 
instance  has  been  found  in  which  one  of  the  resulting 


94  CO^yERSATIONS  ON  CHEMISTRY. 

substances  weighed  less  than  the  oxygen  it  contained. 
All  weighed  more. 

P.  Then  oxygen  can  only  form  compounds  ? 

M.  Yes,  and  no  constituents  of  oxygen  are  known. 
Substances  of  this  sort  are  called  elements.  What  is 
an  element? 

P.  A  substance,  all  the  products  of  change  of  which 
weigh  more  than  it  does  itself. 

M.  Quite  right.  It  can  also  be  said  that  an  element 
is  a  substance  of  which  no  constituents  are  known.  But 
this  definition  is  not  so  clear,  because  it  must  first  be 
known  what  a  constituent  is. 

P.  But  I  learned  before  that  an  element  was  an  unde- 
composable  substance' 

M.  It  means  the  same  thing.  The  changing  of  a 
substance  into  its  constituents  is  called  decomposition. 
Because,  out  of  a  single  thing,  several  different  ones 
arise,  such  a  process  is  called  decomposition. 

P.  Now  I  understand.  But  to  decompose  means  to 
separate  what  is  already  there,  not  to  change  it. 

M.  It  is  like  this:  If  a  definite  amount  of  mercury 
and  oxygen  has  changed,  or  united  into  mercuric  oxide, 
it  is  true  that  the  mercury  and  oxygen  have  vanished, 
but  they  can  be  obtained  out  of  it  again  at  any  time. 
And  exactly  the  same  amount  of  each  constituent  is 
obtained  as  was  originally  there.  You  can  look  at  it 
in  this  way :  as  if  both  the  constituents  in  the  compound 
were  still  really  present,  and  had  hidden  themselves,  as 
it  were,  when  they  combine  with  each  other.  Hence 
the  expressions  decompose  and  combine. 

P.  Yes;  which  is  true  then?  Are  the  constituents 
really  in  the  compound  still,  or  not? 

M.  You    asked    that    question    without    thinking.     A 


ELEMENTS.  95 

compound  isn't  a  bag  or  box  in  which  something  can  be 
*'in."  If  you  understand  by  "in"  that  by  suitable 
means  they  can  always  be  taken  out  of  the  compound, 
they  are  in  it.  But  if  you  mean  that  they  are  hidden 
away  somehow  in  the  compound  with  all  their  properties, 
that  wouldn't  be  clear  and  would  be  misleading.  You 
know  now  what  I  mean  when  I  say  oxygen  is  an 
element. 

P.  Are  there  more  elements? 

M.  Certainly,  mercury  is  one  too.  Sulphur,  iron,  tin, 
lead,  and  copper  are  also  elements.  There  are  altogether 
about  seventy- five  different  elements.  Here  is  a  table 
of  elements  (see  on  the  next  page);  if  you  look  through 
them  you  will  see  some  friends.  But  most  of  them 
are  unknown  to  you.  A  great  many  of  them  also  are 
very  rare,  that  is,  the  substances  out  of  which  they 
can  be  procured  are  rarely  found. 

P.  Can't  the  rare  elements  be  made  out  of  other  sub- 
stances which  are  more  frequently  found? 

M.  No,  that  can  never  be  the  case.  A  given  com- 
pound can  only  be  divided  up  in  one  way  into  elements, 
that  is,  from  every  substance  only  definite  elements 
can  be  obtained,  and  however  one  may  try,  the  same 
elements  are  always  found  in  the  same  proportions. 
And  to  make  this  substance  artificially,  just  the  same 
elements  must  be  taken  in  the  same  proportion, 
or  compounds  must  be  made  use  of  from  which 
these  elements  can  be  got,  or  in  which  they  are 
"contained." 

P.  Is  that  another  law  of  nature? 

M.  Yes,  it  is  the  law  of  the  conservation  of  the  ele- 
ments. 

P.  Please  explain  it  a  little  more. 


96 


CON  VERS  A  TIONS  ON  CHEM  IS  TR  Y. 


Aluminium Al 

Antimony Sb 

Argon. Ar 

Arsenic As 

Barium Ba 

Beryllium Be 

Bismuth Bi 

Boron B 

Bromine Br 

Cadmium Cd 

Caesium Cs 

Calcium Ca 

Carbon C 

Cerium Ce 

Chlorine CI 

Chromium Cr 

Cobalt Co 

Copper Cu 

Erbium Er 

Fluorine F 

Gadolinium Gd 

Gallium Ga 

Germanium Ge 

Gold Au 

Helium He 

Hydrogen H 

Indium In 

Iodine I 

Iridium Ir 

Iron Fe 

Krypton Kr 

Lanthanum La 

Lead Pb 

Lithium Li 

Magnesium Mg 

Manganese Mn 

Mercury Hg 

Molybdenum Mo 

Neodymium Nd 


Neon Ne 

Nickel Ni 

Niobium Nb 

Nitrogen N 

Osmium Os 

Oxygen O 

Palladium Pd 

Phosphorus P 

Platinum Pt 

Potassium K 

Praseodymium Pr 

Radium Ra 

Rhodium Rh 

Rubidium Rb 

Ruthenium Ru 

Samarium Sa 

Scandium Sc 

Selenium Se 

Silicon ■ Si 

Silver Ag 

Sodium -. Na 

Strontium Sr 

Sulphur S 

Tantalum Ta 

Tellurium Te 

Terbium Tb 

ThalHum Tl 

Thorium Th 

Thulium Tu 

Tin Sn 

Titanium Ti 

Tungsten W 

Uranium U 

Vanadium V 

Xenon X 

Ytterbium Yb 

Yttrium Y 

Zinc Zn 

Zirconium Zr 


M.  You  know  that  some  time  ago  there  were  chemists 
who  gave  their  whole  hfe  trying  to  make  gold  or  silver 
out  of  lead  or  other  cheap  metals,  without  one  of  them 
succeeding;  they  were  called  alchemists.  Now,  the 
whole  of  alchemy  was  built  upon  the  hope  that  it  was 
possible  to  change  one  element  into  another,  perhaps 
lead  into  gold.  It  could  not  be  foretold  that  this  was 
not  possible;    it  was  only  by  resultless  efforts  continued 


ELEMENTS.  97 

through  centuries,  that  it  was  found  to  be  impossible 
in  the  case  of  gold  and  silver,  and,  later,  in  the  case  of 
all  other  elements. 

P.  Then  the  gold-making  wasn't  so  mad  and  useless 
in  the  long  run? 

M.  Neither  the  one  nor  the  other.  It  wasn't  mad, 
because  it  became  known  that  it  couldn't  be  done.  Only 
the  gold-makers  didn't  work  scientifically,  that  is,  in  an 
orderly  manner,  because  they  only  tried  things  on  chance. 
And  the  final  result — that  elements  could  neither  change 
into  each  other,  nor  the  compounds  of  definite  elements 
into  the  compounds  of  other  elements — was  an  important 
scientific  discovery,  which  made  the  study  of  chemistry 
far  easier. 

P,  I  don't  understand  that. 

M.  Just  suppose  that  if  we  provide  each  element  with 
a  definite  sign,  then  we  can  mark  every  compound  by 
putting  the  signs  of  their  elements  together.  Just  as 
you  make  the  word  "hat"  out  of  only  the  signs  h,  a, 
and  t  put  together,  and  it  can  only  be  divided  up 
into  these  signs,  and  you  can  never  build  up  the  word 
"rose"  out  of  these  signs,  so  compounds  and  elements 
act  in  the  same  way.  In  the  table  of  elements  (page  96) 
there  is  a  sign  like  that,  against  every  name,  that  is  made 
from  the  first  letter  of  the  name,  and  generally  a  second 
letter  as  well.  Every  substance  that  is  on  the  earth 
can  be  represented  by  placing  together  such  signs,  for 
however  many  substances  there  are,  every  one  of  them 
can  be  decomposed  into  elements  only  in  its  own  par- 
ticular way. 

P.  I  see,  it  is  again  one  of  those  laws  which  are  really 
very  simple,  only  you  must  be  accustomed  to  them  first. 

ikf.  You  will  soon  get  accustomed  enough  to  them. 


gS  CONyERSATIONS  ON  CHEMISTRY. 

In  the  mean  time  we  will  take  our  table  of  elements  and 
see  how  much  chemistry  you  know  already  from  daily 
life.  Oxygen  you  know  already;  it  is  a  colourless  gas. 
Hydrogen  is  a  colourless  gas  too,  but  combustible. 

P.  What  is  hydrogen? 

M.  An  element  that  can  be  obtained  from  water. 

P.  Then  isn't  water  an  element? 

M.  No,  it  isn't  in  the  table.  It  is  a  compound  of 
oxygen  and  hydrogen.  You  know  something  about 
nitrogen  too;  it  is  the  other  ingredient  in  the  mixture 
of  ordinary  air.     It  is  also  a  colourless  and  tasteless  gas. 

P.  Yes,  because  air  is. 

M.  Right.  Now  comes  carbon.  It  isn't  a  gas,  but  a 
solid  body.  Ordinary  charcoal  consists  of  carbon,  but 
not  in  the  pure  state.  These  four  elements  are  always 
in  all  living  things,  plants  as  well  as  animals,  and  as 
such  form  a  definite  group.  That  is  the  reason  I  named 
them  first  to  you.  Moreover  they  are  the  type  of  four 
different  groups  of  other  elements. 

P.  What  does  that  mean? 

M.  Among  the  other  elements  there  are  a  number 
which  behave  in  the  same  way  as  oxygen,  while  others 
are  more  like  hydrogen,  others  like  nitrogen,  and  again 
others  like  carbon. 

P.  What  do  you  mean  by  "like"? 

M.  They  have  to  some  extent  similar  physical  prop- 
erties in  an  uncombined  condition  as  so-called  free 
elements.  In  many  cases  also  the  compounds  which 
are  formed  with  a  third  or  fourth  element  are  like  in 
their  properties. 

P.  That  doesn't  appear  to  me  a  definite  reason  foi 
classifying  them. 

M.  Neither  it  is.     But  by  taking  into  consideration 


ELEMENTS.  99 

all  the  properties  of  all  the  compounds  which  can  be 
produced  from  an  element,  so  many  resemblances  and 
differences  turn  up  that  a  chemist  who  knows  the  rela- 
tionships doesn't  find  the  choice  difficult.  As  you  don't 
know  them  yet  you  must  simply  accept  my  classification. 

P.  But  it  appears  to  me  to  be  unscientific  to  accept 
anything  that  I  can't  prove. 

M.  You  will  be  able  to  prove  it  when  you  have  learnt 
enough  chemistry.  Besides  I  won't  use  the  classification 
for  any  scientific  conclusion,  but  only  for  your  own  con- 
venience, so  that  you  can  learn  the  facts  more  easily; 
besides,  such  arbitrary  things  can  be  treated  in  science 
in  an  arbitrary  manner. 

P.  Yes  I  see. 

M.  Now  impress  the  following  names  on  your  mind: 


*  Hydrogen 

*  Oxygen 

*  Nitrogen 

*  Carbon 

*  Chlorine 

*  Sulphur 

*  Phosphorus 

*  Silicon 

*  Bromine 

Selenium 

Arsenic 

Titanium 

*  Iodine 

Tellurium 

Antimony 

Later  on  we  will  study  carefully  only  those  elements 
marked  with  an  asterisk. 

P.  Why  only  these? 

M.  The  others  are  either  too  seldom  found  in  nature, 
or  their  compounds  have  too  little  importance  in  their 
applications.  As  we  can't  learn  nearly  all  that  has  been 
found  out  up  to  the  present  in  chemistry,  we  must  be 
satisfied  with  a  selection.  I  arrange  this  so  that  at  any 
rate  you  learn  the  substances  which  on  account  of  their 
uses,  or  on  account  of  their  sources,  come  most  frequently 
before  our  notice. 

P.  Then  I  am  only  to  learn  a  little  part  of  chemistry? 

M,  There  are  very  few  people  who  know  every  fact 
that  has  been  proved  in  chemistry  up  to  the  present.  I 
shall  try  to  teach  you  those  parts  of  chemistry  that  wiU 


loo  CONVERSATIONS   ON  CHEMISTRY, 

give  you  the  best  conception  of  the  most  important 
relations.  Later  you  can  take  up  a  special  branch, 
which  you  can  learn  as  thoroughly  as  you  wish.  But 
now  we  will  speak  about  the  elements  we  have  chosen. 
I  have  already  told  you  about  hydrogen,  that  it  is  a  colour- 
less, combustible  gas;  but  its  flame  is  quite  pale  and 
gives  very  little  light.  It  is  the  lightest  substance  there 
is,  and  for  thai*  reason  is  used  for  filling  air  balloons. 

P.  Is  there  hydrogen  in  the  little  red  india-rubber 
balloons  that  children  play  with? 

M.  Certainly,  and  if  one  of  these  freshly  filled  balloons 
is  set  fire  to,  the  hydrogen  burns  with  a  puff. 

P.  I  will  try  that  next  time. 

M,  But  don't  hold  it  too  near  your  face,  or  you  may 
burn  yourself,  for  the  flame  is  hot,  and  it  often  goes 
off  with  a  tremendous  bang. — Chlorine  is  a  greenish 
gas,  with  a  very  unpleasant,  pungent  smell.  Perhaps  you 
have  already  smelt  it,  because  a  white  powder  called 
chloride  of  lime  is  often  scattered  on  unpleasant-smelling 
decomposing  matter;  its  smell  is  that  of  the  greatly  diluted 
chlorine. 

P.  Yes,  I  remember;  our  boy  always  strews  it  at  the 
street  corner.     Why  do  people  do  that? 

M.  The  chlorine  destroys  the  bad-smelling  substances 
and  kills  the  offensive  little  germs  or  mould  or  bacteria. 
— Bromine  is  at  ordinary  temperature  a  deep  reddish- 
brown  coloured  liquid,  and  has  a  yellowish-red  vapour 
which  smells  the  same  as  chlorine. 

P.  Ah,  then  that  is  one  of  those  resemblances  of  which 
you  spoke. 

M.  Yes.  Iodine  smells  like  it  too,  only  at  ordinary 
temperature  it  is  a  solid,  shiny,  black  substance,  the 
vapour  of  which  is  violet. 


ELEMENTS.  loi 

P.  I  remember  that  my  throat  was  painted  with  tinc- 
ture of  iodine  once.  Has  that  anything  to  do  with  the 
element  iodine? 

M.  Yes,  it  is  a  solution  of  iodine  in  spirits  of  wine. 
With  that  we  finish  the  first  group.  Of  the  second  you 
already  know  oxygen.  And  sulphur  is  familiar  to  you 
too. 

P.  The  yellow  stuff? 

M.  Sulphur  is  a  solid  substance  of  a  yellow  colour, 
and  burns  with  a  blue  flame. 

P.  And  in  doing  so  gives  off  a  very  bad  smell.  Why 
do  most  substances  in  chemistry  smell  so  queer  and  un- 
pleasant ? 

M.  The  bad-smelling  substances  are  mostly  those 
which  have  a  corrosive  effect  upon  the  inner  skin  of 
the  nose.  If  they  didn't  smell  badly,  we  wouldn't  notice 
anything,  and  we  would  always  have  a  sore  skin  and  a 
cold  in  our  noses.  Chemistry  would  be  a  far  more 
dangerous  thing  to  work  with  than  it  now  is. 

P.  Ah,  that  is  good.  Do  all  poisonous  substances 
smell  nasty? 

M.  First  of  all,  we  can  only  smell  those  substances 
that  change  into  gas  or  vapour,  because  otherwise  they 
would  never  reach  our  noses.  Fortunately  most  poison- 
ous substances  have  a  bad  smell,  especially  the  corrosive 
ones.  Still  there  are  some  poisonous  gases  and  vapours 
which  have  none,  or  only  a  very  faint  smell.  They  are 
especially  dangerous.  We  will  learn  about  one  of  these 
gases  later  on. 

P.  Then  I'll  take  care. 

M,  We  will  go  now  to  the  nitrogen  group.  You  know 
a  little  about  this  already.  It  is  not  poisonous,  because 
we  breathe  it  together  with  the  oxygen  in  the  air.  But 
in  pure  nitrogen,  without  any  oxygen,  animals  must  die, 


I02  CONVERSATIONS  ON  CHEMISTRY. 

because  they  require  oxygen  to  live.     You  know  some- 
thing about  phosphorus  too. 

P.  Yes,  it  is  in  the  heads  of  matches. 

M.  Right.  From  that  you  know  one  of  its  properties. 
It  catches  fire  very  easily;  even  the  heat  resulting  from 
friction  makes  it  do  that.  That  is  why  it  is  used  in 
matches. 

P.  I  saw  in  the  dark  lately,  that  the  heads  of  matches 
shone;  there  was  a  pale-green  light,  and  the  cook  told 
me  it  was  because  the  matches  had  become  damp.  How 
is  that  possible? 

M.  Phosphorus  bums  slowly  if  it  is  left  in  the  air, 
and  in  doing  so  shines  as  you  saw.  So  that  the  small 
quantity  of  phosphorus,  which  is  contained  in  a  match's 
head,  shall  not  burn  of  its  own  accord,  the  phosphorus  is 
mixed  with  gum,  or  hme,  which  dries,  and  forms  a 
covering  that  keeps  oxygen  out.  In  the  damp  this 
covering  is  dissolved,  and  the  phosphorus  comes  in 
contact  with  air. 

P.  Yes;  but  when  I  wet  a  match  in  the  room  later, 
it  didn't  shine. 

M.  That  must  have  been  a  so-called  Swedish  match; 
they  have  no  phosphorus  in  their  heads. 

P.  What  does  phosphorus  itself  look  like? 

M.  Almost  like  wax.  It  is  kept  under  water,  because 
it  burns  slowly  away  in  air,  as  I  said  before.  Since 
it  is  very  poisonous,  it  is  better  I  should  not  give  it  to 
you  in  your  hand. 

P.  How  is  it  made? 

if.  You  think  you  could  make  it  for  yourself  without 
my  permission!  No,  that  isn't  so  easy.  It  is  one  of 
the  ingredients  of  bones,  and  is  separated  in  a  rather 
comphcated  way. 


LIGHT  METALS.  103 

P.  How  can  it  be  in  bones  if  it  is  so  poisonous? 

M.  Phosphorus  as  a  free  element  is  poisonous,  but 
its  compounds  are  not.  There  you  have  another  exam- 
ple of  how  different  elements  and  their  compounds  can  be. 
— ^Now  we  come  to  the  last  group.  Besides  carbon,  which 
you  already  know,  you  must  learn  about  silicon. 

P.  Does  silicon  come  from  the  Latin  silex,  flint? 

M.  Yes;  flint  consists  of  a  compound  of  silicon  and 
oxygen;  it  is  usually  called  silicic  acid.  Quartz,  sand- 
stone, rock  crystals,  and  flint  consist  of  it.  Finally, 
almost  all  rocks  are  compounds  of  sihca,  so  that  the 
element  silicon  is  one  of  the  substances  that  are  found 
in  the  greatest  quantity  on  the  earth's  surface. — Now 
that  will  do  for  to-day.  I  will  only  say  that  the  elements 
mentioned  now  go  under  the  name  of  non-metals. 
They  form  the  larger  division  of  the  elements;  the  other 
consists  of  metals. 

P.  I  think  I've  learned  a  great,  great  deal  to-day. 

M.  That  was  only  a  walk  through  our  future  work. 
The  real  learning  comes  later. 


14.  LIGHT  METALS. 

P.  How  many  different  sorts  of  metals  are  there? 

M.  About  sixty.  As  we  do  not  know  enough  about 
some,  the  number  is  rather  uncertain. 

P.  But  how  can  you  find  your  way  among  such  a 
large  number? 

M.  In  the  same  way  that  you  can  find  your  way  amongst 
the  much  larger  number  of  animals  and  plants:  they  are 
divided  into  groups,  in  which  those  which  resemble 
each  other  are  put  together. 


tC>4  CONVERSATIONS  ON  CHEMISTRY. 

P.  They  do  that  with  animals  and  plants  according 
to  their  shapes  and  organs;  that  can't  be  done  with 
metals. 

M.  That  is  not  quite  right;  the  crystals  which  form 
when  different  elements  are  in  their  solid  state  show 
some  resemblance,  like  the  shapes  of  plants  and  animals. 
But  metals  have  other  properties  which  are  remarkably 
different  among  each  other,  while  organized  beings 
resemble  each  other  pretty  closely;  those  are  their 
chemical  properties  or  their  capacity  to  form  compounds 
with  other  substances.  Besides  that,  their  physical 
properties,  lustre,  colour,  density,  hardness,  and  so  on, 
are  very  different. 

P.  Then  I  must  know  the  properties  of  all  the  elements 
I  am  to  learn  about,  if  I  am  to  understand  and  remember 
their  classification. 

M.  You  need  to  know  first  of  all  only  those  which  lead 
up  to,  and  complete,  the  classification.  At  present  you 
only  need  to  know  that  the  elements  which  I  place  in  one 
group  possess  definite  resemblances  in  their  properties. 

P.  Yes,  that  is  true.  What  properties  are  the  basis 
of  classification? 

M.  They  are  very  different.  It  happens  that  the 
groups  which  have  been  placed  together  because  of  one 
definite  property  are  almost  always  those  which  would 
be  made  because  of  other  properties.  So  the  present 
usual  grouping  is  the  result  of  quite  a  number  of  these 
selections  of  properties.  Those  which,  in  each  group,  have 
similar  properties  will  be  explained  to  you  separately 
later. 

P.  So  there  is  a  perfect  order? 

M.  Almost,  to  the  same  extent,  as  there  is  order  among 
plants    and    animals.    There,    too,  there    are    doubtful 


LIGHT  METALS.  105 

points,  either  because  the  difference  is  too  little  or  because 
different  methods  of  classifying  lead  to  varying  classi- 
fication. 

P.  But  it  can't  be  that  in  such  unchangeable  things 
as  the  properties  of  elements  there  can  be  contradictions  ? 

M,  There  are  no  contradictions  in  the  properties, 
but  the  irregularities  of  the  somewhat  arbitrary  arrange- 
ment that  we  have  made — 

P.  Yes;  then  why  isn't  everything  simply  arranged 
as  in  arithmetic  or  geometry  ? 

M.  For  this  reason:  we  have  only  incomplete  know- 
ledge of  the  properties  of  the  elements.  Most  of  our 
experiments,  for  example,  are  made  at  temperatures 
which  are  not  very  different  from  that  of  a  room,  and 
under  ordinary  atmospheric  pressure.  Our  conceptions 
of  the  properties  of  the  elements  would  be  quite  different 
if  we  knew  how  they  were  affected  by  all  sorts  of  pressures 
and  temperatures. 

P.  Then  the  imperfection  of  the  classification  is  only 
due  to  the  incompleteness  of  our  knowledge? 

M.  That  is  quite  possible,  for  experience  has  shown 
up  till  now  that  a  department  of  science  becomes  clearer 
and  more  easily  surveyed,  the  more  exact  and  all-embrac- 
ing our  knovdedge  becomes.  But  now  we  will  go  back 
to  our  subject.  We  will  divide  metals  into  light  metals 
and  heavy  metals. 

P.  What  is  the  meaning  of  light  metals?  All  sub- 
stances have  weight,  and  so  are  heavy. 

M.  Quite  right.  Those  metals  whose  density  is  less 
than  four  times  that  of  water  are  called  light. 

P.  Why  was  four  made  the  limit? 

M.  Because  the  other  properties  of  metals  are  such, 
that  by  making  a  limit  here,  it  made  their  differences 


106  CONVERSATIONS  ON  CHEMISTRY. 

most  clear.  This  is  a  case  of  the  mutual  help  of  dis- 
tinguishing marks  that  I  mentioned  before. — Light 
metals  fall  into  three  groups:  the  alkali  metals,  the 
metals  of  the  alkaline  earths,  and  the  metals  of  the  earths. 
These  groups  contain  the  following  important  elements: 


Alkali  Metals. 

Metals  of  the  Alkaline  Earths. 

Metals  of  the  Earths. 

Sodium 

Magnesium 

Aluminium 

Potassium 

Calcium 

P.  But  those  are  very  few. 

M.  They  are  by  no  means  all.  But  I  won't  mention  the 
others  just  at  present,  because  either  they  are  so  seldom 
found,  or  have  so  little  importance  in  their  uses,  that 
you  needn't  bother  yourself  about  them  just  now. 

P.  Is  the  aluminium  which  you  have  named  the  well- 
known  white  metal? 

M.  Yes,  and  if  you  have  had  a  piece  of  it  in  your 
hand  you  will  remember  that  it  is  extraordinarily  light. 
It  is,  in  fact,  only  2.7  times  heavier  than  water. 

P.  Yes.  Aluminium  really  is  a  light  metaL  But  is 
it  true  that  it  is  made  out  of  earth? 

M.  It  is  half  true;  only  earth  is  not  a  definite  chemical 
substance,  but  an  accidental  mixture  of  all  sorts  of  rocks 
and  their  products  from  decay  and  time.  But  in  nearly 
all  rocks  and  earth  aluminium  is  found  in  the  form  of 
a  compound  with  oxygen.  The  different  sorts  of  loam 
and  clay  especially  contain  the  element  aluminium. 

P.  Ah,  that  is  why  it  is  called  a  metal  of  the  earth. 
But  if  it  is  so  common,  why  is  it  so  expensive? 

M.  It  isn't  so  very  expensive  now;  one  pound  costs 
about  twenty-five  cents;  that  it  is  so  much  more  expensive 
than  the  substances  it  is  obtained  from  is  because  it 
requires  a  great  deal   of  work  to  separate  it   from  its 


LIGHT  METALS.  107 

compounds.  It  was  hardly  known  before  the  electric  cur- 
rent began  to  be  used.  The  difference  of  price  between 
aluminium  and  its  compounds,  then,  shows  the  greater 
amount  of  work  or  energy  which  is  contained  in  the 
element  aluminium,  than  in  the  compounds  from  which 
it  is  prepared,  and  as  you  know  work  is  never  given  as 
a  present. 

P.  Can  you  get  the  work  out  of  the  aluminium  again? 

M.  Certainly.  Here  is  a  mixture  of  aluminium  with 
a  compound  of  iron,  iron  oxide,  which  you  already  know. 
If  I  light  this  mixture  an  immense  amount  of  heat  is 
given  off,  the  mixture  glows  white  hot,  the  metal  iron  is 
set  free,  and  all  sorts  of  welding  and  melting  can  be  done 
with  the  hot  mass. 

P.  That  is  a  pretty  experiment.  How  was  the  mix- 
ture made? 

M.  Aluminium  powder  and  iron  oxide  are  mixed  in 
the  proportion  of  i  to  3.  Both  substances  must  be  thor- 
oughly dried  beforehand  by  heat.  The  lighting  is  done 
with  a  small  piece  of  magnesium  ribbon  (you  will  soon 
learn  about  magnesium  itself),  which  is  made  to  burn 
by  means  of  a  match.  The  mixture  is  placed  in  a 
clay  crucible,  or  in  a  cavitj,  which  you  can  make  in  a 
dry  brick. 

P.  What  happens  exactly? 

M.  Iron  oxide,  as  you  knew,  is  a  compound  of  iron 
and  oxygen.  If  aluminium  comes  in  contact  with  it 
when  hot,  it  unites  with  the  oxygen,  and  the  iron  is 
separated;  as  through  the  uniting  of  oxygen  with  alumin- 
ium much  more  work  is  set  free  than  is  necessary  to 
separate  oxygen  from  iron,  a  great  amount  is  left  over, 
which  appears  as  heat. 

P.  Is  work  the  same  thing  as  heat? 


toS  CONVERSATIONS  ON   CHEMISTRY, 

M.  In  so  far  as  the  one  can  be  changed  into  the  other. 
Vou  can  tell  that  work  changes  to  heat  because,  by  fric- 
tion, heat  is  obtained.  And  to  overcome  friction,  work 
is  necessary. 

P.  Yes,  now  I  know.  And  a  steam-engine  makes 
work  from  heat. 

,  M.  Right.  But  now  we  must  go  back  to  our  light 
metals.  Of  the  metals  of  the  alkaline  earths  you  prob- 
ably already  know  magnesium. 

P.  Isn't  it  magnesium  that  burns  so  brightly? 

M.  Yes,  magnesium  is  a  light  white  metal,  which  can 
be  lighted,  and  burns  with  a  bright  flame.  It  is  used  when 
a  bright  light  is  required  and  no  electrical  current  is  at 
hand.  For  that  purpose  it  is  generally  made  in  the  form 
of  a  narrow  strip  or  ribbon.  Here  is  such  a  piece  of 
magnesium  ribbon  that  is  brought  in  this  form  into 
commerce.     I  light  it,  and  you  see  how  brightly  it  burns. 

P.  What  is  the  white  ash  and  the  white  smoke  that 
comes  from  it? 

Af.  That  you  ought  to  know  for  yourself.  What  is 
combustion  ? 

P.  A  combination  with  oxygen.  Then  is  the  white 
stuff  an  oxide  of  magnesium? 

M.  Yes.  And  the  strong  light  is  another  example 
that  in  this  combination  between  oxygen  and  magnesium 
there  is  a  great  deal  of  surplus  work  which  shows  itself 
as  light  and  as  heat. 

P.  Then  is  light  a  sort  of  work? 

M.  Yes,  certainly.  You  know  that  plants  grow  and 
increase  in  light  and  form  wood,  leaves,  and  so  on.  The 
wood  you  can  burn  and  obtain  heat  from,  as  a  sign  that 
there  is  work  in  it.  This  work  has  come  from  the  Hght 
of  the  sun,  because  plants  can  only  grow  in  light. 


LIGHT  METALS,  109 

P.  Where  is  magnesium  found? 

M.  Like  aluminium  it  must  be  obtained  from  its 
compounds  by  means  of  electric  work.  In  nature, 
compounds  of  magnesium,  generally  with  oxygen,  occur 
in  very  large  masses.  Dolomite,  which  forms  large 
mountains,  is  rich  in  magnesium  compounds;  they  also 
occur  in  nearly  all  rocks. 

P,  What  is  magnesia  that  is  used  as  medicine?  Has 
it  anything  to  do  with  the  metal  magnesium? 

M.  Yes,  it  is  magnesium  oxide,  the  same  substance 
which  is  formed  on  burning  the  metal.  The  medicine, 
Epsom  salts,  is  also  a  compound  of  magnesium.  All 
these  substances  you  will  learn  more  about  later  on. 

P.  I  should  really  like  to  have  heard  more  about 
magnesium:  there  are  so  many  sorts  of  things  connected 
with  it. 

ilf .  You  will  find  the  same  thing  with  other  metals. 
Calcium,  for  example,  as  a  metal,  is  very  little  known, 
because  it  takes  far  more  work  than  magnesium  does 
to  separate  it  from  its  compounds,  and  it  bums  far  more 
easily. 

P.  Why  should  I  learn  about  it  now? 

M.  Because  its  compounds  are  extraordinarily  exten- 
sive; it  belongs  to  the  elements  in  which  the  earth's 
surface  .is  richest.  Limestone,  of  which  large  mountains 
and  countries  consist,  is  one  of  its  compounds;  chalk 
and  marble  are  the  same  compound  in  rather  different 
forms. 

P.  But  chalk,  marble,  and  limestone  are  surely 
different ! 

M,  Yes,  in  their  outward  appearance.  But  if  I  take 
a  small  piece  of  the  three  substances,  and  pour  hydro- 
chloric acid  over  them,  they  behave  in  the  same  way; 


no  CONyERSATIONS  ON  CHEMISTRY, 

they  froth  up  and  let  a  gas  escape.  And  the  resulting 
solutions  give,  in  the  same  way,  a  white  precipitate  if 
I  add  dilute  sulphuric  ^cid.  And  there  are  a  great 
number  of  other  reactions  which  always  occur  which- 
ever of  the  three  minerals  I  use.  Their  difference  is  only 
that  dialk  consists  of  far  smaller  particles  than  the  other 
two,  and  that  limestone  contains  additional  impurities, 
which  make  its  colour  appear  grey.  But  marble  also 
often  contains  impurities  and  appears  red,  sometimes 
black.  So  the  three  are  only  different  physically;  chem- 
ically they  are  the  same. 

P.  Are  there  other  compounds  of  calcium? 

M.  Innumerable.  By  pouring  water  on  the  burnt  lime 
which  is  got  by  strongly  heating  limestone,  it  heats 
itself  and  swells  up,  and  with  more  water  forms  milk 
of  lime,  which,  mixed  with  sand,  is  used  as  mortar. 
Gypsum  and  cement  are  also  compounds  of  calcium. 

P.  I'd  like  to  learn  more  about  them  too. 

M.  Again,  you  must  wait  till  later  on  for  them;  other- 
wise we  won't  get  through  our  talk.  Now  we  have 
still  the  first  group — the  alkali  metals — to  consider.  Look, 
in  this  glass  there  is  sodium. 

P.  It  looks  white  like  silver.  But  why  is  the  glass 
sealed  ? 

M.  Because  sodium  combines  even  at  ordinary  tem- 
perature with  the  oxygen  of  the  air.  As  no  air  can  get 
in  through  the  glass  the  metal  remains  unchanged,  and 
its  white  colour  and  silver  appearance  can  be  recognized. 
These  grey  pieces  are  also  sodium. 

P.  But  they  look  quite  different! 

M.  That  is  only  on  the  surface,  where  the  compound 
of  oxygen  has  formed.  If  I  cut  off  this  layer  with  a 
knife,  the  shining  metal  will  be  exposed. 


LIGHT  METALS,  III 

P.  But  it  will  soon  be  grey  again! 

M.  Yes,  it  will  combine  with  the  oxygen  in  the  air. 

P.  What  sort  of  liquid  is  the  piece  of  sodium  in? 

M.  It  is  ordinary  petroleum.  I  told  you  before  that 
it  was  made  of  hydrogen  and  carbon;  it  contains  no 
oxygen.  That  is  why  sodium  can  be  kept  in  it,  and  is 
protected  from  forming  a  compound  with  oxygen. 

P.  Then  can  sodium  take  oxygen  out  of  a  compound? 

M,  Certainly.  I  throw  a  piece  of  sodium  into  water. 
It  becomes  hot,  melts,  and  the  ball  dances  about  on  the 
water,  always  getting  smaller.  Now  take  care,  a  little 
explosion  will  follow.  See,  now  it  is  over,  and  all  the 
sodium  has  vanished. 

P.  Where  has  it  gone? 

M.  It  has  united  with  the  oxygen  in  the  water,  and 
has  become  an  oxide,  which  has  dissolved  in  the  water. 

P.  Is  this  oxide  found  in  nature? 

M.  No,  it  can  only  be  artificially  made.  But  there 
is  another  compound  that  is  found  in  nature.  Ordinary 
salt  is  a  compound  of  sodium. 

P.  With  what? 

M.  With  chlorine. 

P.  That  can't  be  true,  surely. 

M.  Why  not? 

P.  Sodium  is  such  an  acrid  stuff,  and  chlorine  too, 
and  yet  their  compound  makes  common  salt  which  we 
can  eat. 

M.  You  have  guessed  wrong  again,  as  if  the  ele- 
ments were  contained  as  such  in  their  compounds.  That 
common  salt  is  a  compound  of  sodium  and  chlorine 
tells  you  no  more  than  that  salt  can  be  made  with  both, 
and  vice  versa,  both  elements  out  of  salt. 

p.  Is  that  really  possible? 


112  CONVERSATIONS  ON  CHEMISTRY. 

M.  You  shall  see  it  for  yourself  later  on. 

P.  I  can  hardly  wait  to  see  and  learn  about  all  these 
wonderful  things. 

M.  At  present  we  must  speak  about  the  last  light  metal 
potassium.     Here  is  a  glass  tube  with  potassium. 

P.  It  looks  just  like  sodium. 

M.  Yes,  and  behaves  in  a  similar  way.  If  I  take 
a  piece  out  of  the  oil  where  it  is  kept  and  throw  it 
into  water  the  effect  is  so  strong  that  a  violet  flame  is 
the  result. 

P.  Then  potassium  won't  appear  as  a  metal  in  nature  ? 

M.  No!  if  there  had  been  any  to  be  found,  it  would 
have  seized  all  the  water  to  be  had,  and  changed  into 
a  compound  with  oxygen. 

P.  What  are  the  compounds  of  potassium  ? 

M.  There  are  a  great  many.  Among  the  substances 
that  you  know  I  will  name  saltpetre.  Further,  potassium 
is  an  ingredient  of  many  minerals.  Ordinary  red  felspar 
contains  potassium.  There  are  potassium  compounds  in 
the  earth  from  rocks,  and  they  are  taken  up  by  plants, 
which  need  potassium  to  enable  them  to  live.  For  that 
reason  potassium  compounds  are  found  on  the  ashes  of 
plants.  They  remain  behind  on  burning,  as  they  are 
not  volatile.  They  can  be  separated  from  the  ashes 
with  water,  and  'by  evapoilating  the  water,  are  obtained 
in  solid  form.  The  white,  salty-looking  substance  which 
is  so  obtained  is  called  potash. 

P.  I  would  like  to  make  that. 

M,  It  is  quite  easy ;  you  only  need  to  stir  up  wood-ash 
with  water  and  pour  it  through  a  filter  (see  page  15). 
Then  a  clear  liquid  runs  through,  which  tastes  like  soap, 
and  leaves  a  white  or  grey  salt  behind,  if  it  is  put  in  a 
saucer  in  a  warm  oven.      But   take   care  that  you  use 


HEAVY  METALS.  1 13 

only  the  ash  of  wood,  and  not  that  of  coal,  because  that 
doesn't  contain  potash. 

P.  I  have  learned  so  much  to-day  that  I'm  afraid  I 
shall  never  remember  it  all. 

M,  All  that  we  have  been  speaking  about  will  come 
again  later  on  when  we  learn  the  compounds  of  separate 
elements.  To-day  I  only  showed  you  that  you  know 
quite  a  lot  of  chemistry,  that  is  to  say,  many  substances 
which  you  have  noticed  in  daily  life.  You  must  certainly 
gain  first  orderly  knowledge  of  substances  and  their 
behaviour,  that  is,  real  scientific  knowledge. 

P.  I  shan't  fail  for  want  of  diligence  and  attention. 


15.  HEAVY  METALS. 

M.  To-day  we  begin  to  talk  about  heavy  metals. 
Among  these  are  the  ones  that  have  been  longest  known, 
such  as  copper,  gold,  tin,  lead,  and  iron. 

P.  Why  were  just  these  known  first? 

M.  Gold  is  found  as  such  in  the  earth.  Copper,  tin, 
and  lead  are  very  easily  separated  from  their  ores,  so 
that  it  was  possible  to  obtain  them  at  an  early  epoch 
without  any  great  experience  or  skill.  Iron  came  into 
use  much  later,  as  it  was  more  difficult  to  obtain.  But 
we  will  make  a  table  again.  And  here  also  I  will  only 
bring  before  you  the  most  important  metals: 


Iron 

Nickel 

Copper 

Silver 

Gold 

Manganese 

Chromiutn 

Leac 

Tin 

Platinum 

Cobalt 

Zinc 

Mercury 

P.  I  know  almost  all  of  these. 

M.  You  won't  know  much  about  manganese.    It  is 
a   metal  that    is  very  like    iron,  and  you  have  learned 


114  CONIFERS  AT  IONS  ON  CHEMISTRY. 

about  its  compound  with  oxygen  in  one  of  our  earlier 
experiments.  It  is  the  substance  which  we  used  when 
preparing  oxygen  from  potassium  chlorate  to  make  it 
come  off  more  easily. 

P.  Cobalt  is  blue;  is  it  an  element  too? 

M.  No,  the  blue  colour  is  that  of  a  compound  of  the 
element  cobalt.  Cobalt  is  also  like  iron,  but  keeps  better 
in  air,  and  doesn't  rust  like  iron.     You  know  nickel? 

P.  Yes. 

M.  Some  coins  are  made  of  nickel.  Besides,  cook- 
ing-utensils are  made  of  it.  The  metal  is  far  whiter 
than  iron,  almost  like  silver,  and  remains  bright  in 
damp  air  without  rusting.  It  is  hard,  and  is  difficult 
to  melt.     For  that  reason  it  is  a  fairly  valuable  metal. 

P.  What  happens  to  iron  when  it  rusts? 

M.  It  combines  with  the  oxygen  of  the  air  and  with 
water.  Therefore  iron  keeps  much  better  in  dry  air 
than  in  damp  air. 

P.  What  does  nickel-plating  mean? 

M.  It  means  covering  over  with  nickel.  With  the  help 
of  an  electric  current  the  metal  can  be  deposited  from 
solutions  of  nickel  compounds  on  to  any  sort  of  metal 
object.  As  nickel  keeps  so  well  in  air,  these  covered 
or  nickel-plated  objects  keep  better  than  without  this 
covering. 

P.  I  don't  know  chromium  at  all. 

M.  I  won't  tell  you  much  about  this  element  yet.  It 
is  whiter  than  iron,  very  hard,  and  melts  with  great  diffi- 
culty. Many  of  its  compounds  are  brightly  coloured 
and  so  are  used  as  colours  for  pictures  and  painting. 
But  you  know  zinc  ? 

P.  Is  it  the  white  or  light-grey  metal  of  which  roof- 
gutters  and  whole  roofs  and  bath-tubs  are  made? 


HE/tyy  METALS.  115 

M.  Yes;  it  is  much  softer  and  more  easily  melted 
than  the  other  metals  which  were  mentioned  before. — 
We  now  come  to  the  copper  group.  You  already  know 
that  metal  quite  well. 

P.  Yes,  and  I  know  lead  too ;  it  is  so  heavy. 

M.  Its  density  is*ii.4.  It  melts  very  easily,  and  is  soft. 
Most  metals  with  low  melting-points  are  soft. 

P.  And  vice  versa  ? 

M.  No,  it  doesn't  hold  the  reverse  way.  Gold  and 
silver  are  fairly  soft,  but  have  a  high  melting-point. 
But  it  holds  again  for  tin:   tin  is  rather  soft. 

P.  And  it  can  be  very  easily  melted.  We  did  it  on 
New  Year's  day,  and  poured  it  into  water.  What  made 
the  crinkled  shapes  that  we  got? 

M.  You  should  be  able  to  answer  that  for  yourself. 
Tin  melts  at  235°.  What  will  happen  if  you  pour  melted 
tin  into  water? 

P.  The  water  will  begin  to  boil.  Now  I  understand 
it:  the  water  makes  steam,  and  swells  the  Hquid 
metal. 

M.  Right.  And  it  hardens  when  it  comes  in  contact 
with  the  remaining  water. — ^What  do  you  know  of  mer- 
cury? 

P.  That  it  is  liquid  at  the  ordinary  temperature. 

M.  It  is  the  only  metal  that  has  this  property.  It 
isn't,  however,  the  only  liquid  element,  for  bromine  at 
ordinary  temperatures  is  also  liquid. — You  know  silver 
too? 

P.  Yes,  from  silver  coins  and  teaspoons. 

M.  Mercury  and  silver  are  counted  as  precious  metals, 
and  so  are  gold  and  platinum  in  the  next  group. 

P.  Why  are  they  called  that?  Because  they  are  so 
expensive  ? 


Il6  CONVERSATIONS  ON  CHEMISTRY. 

M'  Not  exactly  for  that  reason,  as  there  are  other 
much  rarer  elements,  which  are  much  more  costly,  and 
yet  are  not  called  precious.  No,  they  are  called  so 
because  they  remain  bright  when  heated,  and  don't 
become  black  and  ugly  like  other  metals. 

P.  But  why? 

M.  That  you  must  answer  for  yourself.  I  have 
already  told  you  what  happens  to  iron  when  it  is  heated 
in  air. 

P.  Yes,  it  combines  with  oxygen,  and  the  other  metals 
will  do  the  same.  Can't  the  precious  metals  combine 
with  oxygen? 

M.  Certainly.  Their  oxides  are  also  known.  But 
they  have  the  property  that  when  heated  they  decom- 
pose into  metal  and  oxygen.  I  showed  it  to  you  once 
before  with  mercury. 

P.  Oh,  so  that  is  why  their  oxides  can't  be  formed  by 
heating  the  metal,  as  they  would  at  once  decompose. 

M.  Right.  It  requires  work  to  make  these  metals 
combine  with  oxygen,  and  heat  alone  can't  perform  this 
work. 

P.  Do  the  precious  metals  form  no  compounds? 

M.  Yes,  some  can  be  obtained  if  the  precious  metals 
are  treated  with  substances  which  yield  work  on  com- 
bination. Sulphur,  for  example,  does  so  with  silver  and 
mercury. 

P.  Can  I  see  it? 

M.  Certainly.  I  put  a  drop  of  mercury  in  a  mortar 
and  add  some  sulphur  to  it.  Then  I  rub  both  together. 
What  do  you  see? 

P.  It  is  all  becoming  black.  Now  there  is  a  fine 
black  powder,  like  soot.     What  is  it? 

M.  It  is  a  compound  of  sulphur  and  mercury.     In 


MORE  ABOUT  OXYGEN.  li7 

the  same  way  silver  can  be  combined  with  sulphur. 
Rub  some  sulphur  with  a  cork  on  a  silver  coin. 

P.  The  silver  is  becoming  brown  and  blackish  grey. 

M.  There  again  is  another  combination  of  both 
elements.  Both  metals  unite  directly  in  the  same  way 
with  chlorine,  bromine,  and  iodine. 

P.  Aren't  these  precious,  then? 

M.  No.  But  gold  and  platinum  are  still  more  precious, 
as  they  don't  combine  with  sulphur  by  rubbing  them 
together. 

P.  Don't  they  combine  with  anything? 

M.  Yes,  they  can  combine  with  chlorine.  But*  this 
compound  decomposes  into  elements  again  on  heating, 
just  as  you  saw  with  mercuric  oxide.  We  will  stop  with 
that  for  to-day. 

P.  Chemistry  is  a  tremendously  large  subject. 


16.  MORE  ABOUT  OXYGEN. 

M.  To-day  we  will  learn  more  about  oxygen. 

P.  I  know  about  it  already. 

M.  Only  ver)^  superficially,  for  you  know  only  a  very 
small  part  of  what  is  known  about  it.  And  what  I  am 
going  to  tell  you  is  only  a  little  part  of  what  is  known. 

P,  But  you  know  all  about  it? 

M.  No,  I  don't  think  there  is  a  single  man  who  really 
knows  all  that  is  known  about  oxygen. 

P.  1  don't  understand  that.  If  no  one  knows  it,  then  it 
isn't  known. 

M.  One  man  knows  one  part,  another  man  knows 
another,  so  that  the  knowledge  is  present  in  somebody's 


1 1 8  CONyERSA  TIONS  ON  CHEMISTR  Y, 

mind,  but  not  all  in  the  same  person's.  Besides,  almost 
all  is  to  be  found  in  books,  and  is  accessible  for  every  one 
who  wishes.  From  time  to  time  a  man  is  found  who 
discovers  as  much  as  possible  about  it,  and  puts  it  all 
together  in  a  particular  book,  to  save  others  the  trouble  of 
searching.  But  he  can  only  give  extracts,  and  so  one 
who  for  some  reason  wishes  to  learn  exactly  what  is 
known  of  the  subject  must  look  over  the  books  himself, 
or  by  experiment  arrive  at  the  desired  knowledge. 

P.  Is  everything  right  that  is  found  in  books? 

M.  Most  of  it ;  and  when  there  is  anything  wrong,  it 
is  no  intentional  error,  but  the  author  for  some  reason 
made  a  mistake.  A  most  remarkable  and  praiseworthy 
thing  in  scientific  literature  is  that  almost  every  word 
is  written  conscientiously. 

P.  But  if  someone  has  made  an  oversight  and  written 
something  wrong,  the  error  would  remain  there  forever. 

M.  Only  until  it  is  contradicted  by  some  other  fact 
that  is  found.  Then  it  is  seen  on  which  side  the  fault 
lies,  and  after  that  one  can  generally  find  out  how  the 
error  came.  But  now  we  will  go  back  to  oxygen.  You 
remember  how  we  made  it  before? 

P.  Yes,  from  a  white  salt.     What  is  it  called? 

M.  Potassium  chlorate.  It  contains  about  two  fifths 
of  its  weight  of  oxygen,  which  it  gives  up  when  gently 
heated,  especially  if  a  little  oxide  of  iron  or  of  manganese 
be  added. 

P.  You  told  me  that  already  (page  114)  but  it  strikes 
me  as  so  remarkable  that  I  should  like  to  see  it.  Can 
you  show  me  how  iron  oxide  makes  it  easier  for  the 
oxygen  to  come  off? 

M.  Certainly.  I  am  melting  a  Httle  potassium  chlorate 
in  a  test-tube.    What  do  you  see? 


MORE  /iBOUT  OXYGEN.  1 19 

P.  It  melts;  now  it  has  become  as  clear  as  water; 
now  quite  small  bubbles  are  rising. 

M,  These  are  traces  of  oxygen.  Now  I  take  the 
lamp  away  from  the  glass,  and  add  a  little  oxide  of  iron 
to  it. 

P.  It  froths  like  soda-water.  Does  the  salt  begin  to 
boil? 

M.  No,  oxygen  comes  off  suddenly.  If  I  put-  in  a 
burning  splinter,  it  catches  fire.  You  know  that  is  the 
test  for  oxygen.  You  see  that  even  though  the  salt  has 
cooled  a  little  on  taking  away  the  flame,  the  oxygen 
comes  off  much  more  quickly  on  adding  the  oxide  of 
iron. 

P.  That  is  really  very  curious.     Why  does  it  happen? 

M.  The  oxide  of  iron  has  acted  like  oil  on  a  rusty 
machine,  or  like  a  whip  on  a  horse. 

P.  I  don't  understand  that. 

M.  You  are  not  the  only  one.  It  is  a  fact  that  many 
chemical  processes  which  go  very  slowly  of  themselves 
can  be  accelerated  by  adding  other  substances  to  them, 
even  though  the  added  substances  undergo  no  permanent 
change.  The  investigation  of  the  question  why  these 
accelerations,  which  are  ascribed  to  catalytic  action,  really 
take  place  is  a  difficult  scientific  problem,  and  perhaps 
in  a  few  years  I  may  be  able  to  give  you  an  answer.  In 
the  mean  time  we  will  use  this  fact  as  a  convenient  help. 

P.  When  I  know  more  I'll  try  to  find  out  the  reason 
of  catalytic  actions. 

M.  That  is  a  good  plan.  But  now  we  will  make  some 
oxygen.  You  know  already  how  it  is  done.  First  I 
will  place  this  flask  filled  with  water  here,  for  we  must 
first  expel  the  air  from  the  flask  by  oxygen  before  I  col- 
lect it. 


120  CONyERSATlONS  ON  CHEMISTRY. 

P.  But  you  will  lose  some  oxygen  in  that  way. 

M.  That  doesn't  matter ;  if  we  want  it  pure,  we  must 
make  up  our  mind  to  that.  You  will  always  meet  with 
that  same  difficulty  in  future.  Now  I  begin  to  heat, 
and  you  see  that  soon  bubbles  rise  out  of  the  glass  tube. 
Now  place  the  flask  on  the  stand,  but  take  care  that  you 
always  keep  its  mouth  under  water  or  else  air  will  enter. 

P.  How  quickly  it's  going!. 

M.  Yes;  it  will  be  better  to  take  the  flame  away  for 
an  instant.  Now  fill  an  empty  flask  with  water  and 
have  it  ready. 

P.  But  how  can  I  turn  it  upside  down  without  letting 
the  water  run  out? 

M.  Hold  your  thumb  on  the  mouth. 

P.  My  thumb  is  too  small. 

M.  Then  take  your  hand  or  a  piece  of  cardboard,  or 
anything  flat.     The  best  thing  is  a  cork  that  fits  it. 

P.  Now  the  first  flask  is  full  of  oxygen. 

M.  I  close  it  under  water  with  a  cork,  and  can  take 
it  out  and  put  it  aside. 

P.  Why  do  you  put  it  upside  down? 

M.  Because  generally  the  cork  doesn't  fit  tight,  and 
the  water  then  prevents  the  oxygen  from  coming  out. 
Now  the  second  flask  is  nearly  full;  get  another  flask 
ready. 

P.  I  didn't  think  that  so  much  oxygen  could  come  out 
of  so  small  a  quantity  of  salt ;  the  sixth  large  flask  is  half 
full,  but  it  has  stopped  coming  now. 

M.  Yes.  Now  we  will  take  the  tube  out  of  the  water; 
if  we  didn't,  the  water  would  rise  up  into  the  flask  and 
break  the  hot  glass. 

P.  What  a  lot  of  things  there  are  to  think  about! 

M.  Yes,   the  art  of  experimenting  is  not  to  require 


MORE  ABOUT  OXYGEN.  121 

to  think  about  such  things,  but  to  do  them  involuntarily. 
Now  we  will  do  what  we  had  to  put  off  before;  we  will 
calculate  the  density  of  oxygen. 

P.  Calculate?    But  we  must  first  measure  it. 

M.  The  measurements  are  already  made.  I  used  lo 
grams  of  potassium  chlorate;  it  contains  about  4  grams 
of  oxygen;  more  correctly  3.9.  Our  flasks  are  each 
half  a  litre  or  500  cubic  centimetres  in  capacity,  as  you  can 
see  by  the  500-mark  which  is  stamped  upon  the  bottom 
of  each.  We  have  thus  collected  somewhat  less  than 
3  litres  of  oxygen,  so  each  litre  weighs  in  round  numbers 
1.3  grams,  and  each  cubic  centimetre  0.0013  gram,  so 
(see  page  48)  the  density  of  oxygen  is  equal  to  0.0013. 

P.  I  shouldn't  have  thought  the  calculation  could  have 
been  made  so  easily. 

M.  It  was  easily  made,  but  it  was  not  exact.  I  wanted 
to  show  you  how  to  arrive  at  a  knowledge  of  such  values. 
It  wasn't  my  intention  to  make  an  accurate  measure- 
ment. 

P.  One  thing  more.  You  said  that  the  weight  of  the 
oxygen  from  10  grams  of  potassium  chlorate  was  3.9 
grams,  but  not  how  you  found  it  out. 

M.  That's  not  difficult.  You  weigh  the  test-tube  with 
the  chlorate  before  heating,  and  then  afterwards. 

P.  I  see  it  now.  The  loss  of  weight  is  equal  to  the 
weight  of  the  oxygen  that  has  come  off. 

M.  Yes.  Here  you  have  an  application  of  the  law  of 
the  conservation  of  weight. 

P.  So  I  have  used  a  law  of  nature  without  knowing 
it.  What  is  the  use  of  stating  these  laws  of  nature  when 
you  use  them  without  knowing  them? 

M.  It  was  an  accident  that  you  used  it  rightly.  It 
is  just  as  easy  to  make  a  wrong  use  of  them,  and  in  order 


122  CONVERSATIONS  ON  CHEMISTRY. 

to  avoid  that  the  law  must  be  expressed  and  used  inten- 
tionally. This  is  troublesome  at  first,  but  later  on,  if  my 
teaching  makes  the  right  impression  on  you,  whenever 
you  learn  anything  new,  you  will  find  it  necessary  to 
state  it  as  a  law  of  nature. 

P.  I  don't  think  I  shall  ever  get  as  far  as  that. 

M.  We  mustn't  forget  that  we  are  speaking  of  oxygen 
all  the  time.  When  we  collected  it  over  water,  did  you 
notice  anything  strange? 

P.  I  don't  think  I  did. 

M.  The  bubbles  of  oxygen  rose  through  the  water 
without  diminishing  in  size.  That  is  a  proof  that 
oxygen  is  insoluble,  or  very  sparingly  soluble  in 
water. 

P.  Can  gases  dissolve  in  water,  then? 

M.  Certainly.  You  have  an  example  in  soda-water. 
As  long  as  it  is  in  the  bottle  it  looks  quite  clear,  but  when 
you  pour  it  out  a  quantity  of  gas  escapes  which  was 
dissolved  before. 

P.  Yes,  I  have  seen  that.  But  why  does  the  gas  escape 
when  you  pour  it  out? 

M.  Gases  dissolve  iri  water  and  other  liquids  more 
readily  at  high  than  at  low  pressures.  In  the  bottle  the 
solution  is  at  pretty  strong  pressure,  and  when  the  bottle 
is  opened  the  pressure  is  reheved,  so  that  the  gas  escapes. 

P.  Ah,  that  is  the  reason  why  it  pops  and  foams. 
What  sort  of  gas  is  it? 

M.  It  is  carbonic  acid  gas,  the  same  gas  which  is 
produced  when  carbon  burns  in  air  or  oxygen.  We 
shall  get  to  know  more  about  it  afterwards. 

P.  Then  we  ought  to  be  able  to  make  carbonic  acid 
gas  out  of  smoke. 

M.  That  doesn't  work;  for  in  smoke  the  gas  is  mixed 


MORE  ABOUT  OXYGEN.  123 

with  much  nitrogen  of  the  air,  and  besides,  it  contains 
disagreeably-smelling  stuff  from  the  coal. 

P.  I  only  meant  it  as  a  joke. 

M.  But  the  proposal  is  quite  a  sensible  one.  If 
carbonic  acid  gas  were  an  expensive  gas,  it  would  be 
worth  considering  whether  it  couldn't  be  separated 
from  the  mixture  and  purified.  But  because  such  a 
separation  costs  trouble  and  money,  the  question  is 
asked,  can  it  not  be  made  in  a  cheaper  way?  The 
answer  to  that  question  is  the  foundation  of  the  chief 
part  of  chemical  industry.  But  we  will  go  back  to 
oxygen.  It  is  very  sparingly  soluble  in  water;  while 
water  dissolves  its  own  volume  of  carbonic  acid  gas, 
it  dissolves  only  about  a  fiftieth  of  its  own  volume  of 
oxygen. 

P.  But  if  it  is  more  strongly  compressed? 

M.  It  remains  the  same.  If  a  gas  is  compressed  more 
strongly,  more  goes  into  the  same  volume,  and  water 
dissolves  exactly  as  much  more.  On  the  other  hand, 
the  proportion  varies  with  the  temperature;  the  higher 
the  temperature  the  less  gas  dissolves.  What  do  you 
notice  when  fresh  spring-water  is  allowed  to  stand  in  a 
room  for  some  time? 

P.  Do  you  mean  the  little  bubbles  of  air  that  stick 
to  the  side  of  the  glass  ? 

M.  Yes,  that  is  what  I  mean.  When  the  cold  water 
which  is  saturated  with  gas  warms  up,  part  of  it  must 
escape,  and  it  does  so  in  the  form  of  bubbles,  which 
gradually  grow  larger,  and  finally  separate  and  rise. 
We  have  learnt  something  about  the  behaviour  of  oxygen 
when  it  is  kept  in  a  flask  by  itself,  and  when  it  is  brought 
together  with  other  bodies.  Now  we  will  get  to  know 
about  it  in  the  free  state. 


124  CONyERSATIONS  ON  CHEMISTRY. 

P.  I  am  curious  to  hear  about  that. 

M.  You  know  that  it  is  a  constituent  of  air,  and  indeed 
the  most  active.  The  other  constituent  is  called  nitrogen, 
and  animal  life  cannot  survive  in  it,  nor  can  flames  burn 
in  it,  but  go  out.  As  air  penetrates  everyv^here,  so  oxy- 
gen can  penetrate  everywhere,  and  it  combines  with  sub- 
stances which  are  present;  that  has  happened  as  long  as 
our  earth  has  been  in  the  same  condition  as  it  is  now; 
that  is,  for  thousands  and  thousands  of  years.  The 
consequence  is,  that  everywhere  on  the  earth's  surface 
compounds  of  oxygen  are  to  be  found.  Most  of  the 
substances  which  we  know  contain  oxygen.  Compounds 
of  other  elements  with  oxygen  are  called  oxides.  The 
word  oxygen  comes  from  the  Greek  and  means  an  acid 
substance. 

P.  What  has  it  to  do  with  acid  ?     It  isn't  acid,  surely  ? 

M.  It  occurs  in  many  acid  substances.  It  was  formerly 
believed  that  its  presence  was  necessary  to  the  existence 
of  acid  substances,  but  that  has  subsequently  been  found 
to  be  erroneous. 

P.  Why  did  they  keep  the  false  name? 

M,  Nobody  thinks  about  it  now,  so  it  doesn't  matter. 
But  we  will  leave  the  name  and  go  back  to  the  thing. 
You  know  that  by  burning  fuel  we  not  only  warm  our 
houses  in  winter,  but  we  drive  our  machines,  lift  weights, 
in  fact,  do  all  sorts  of  work  that  we  require  to  do.  Burn- 
ing means  union  with  oxygen.  How  does  it  happen  that 
that  enables  us  to  do  work? 

P,  Oh,  I  know  that  from  our  former  lesson.  Burn- 
ing is  a  chemical  process  by  which  energy  is  set  free. 

M.  I  am  glad  you  remembered  it.  Now  I  will  give 
you  a  riddle.  How  does  it  happen  that  the  coal  doesn't 
bum  in  our  cellar? 


MORE  ABOUT  OXYGEN,  125 

P.  Because  there  is  nothing  there  to  set  it  on  fire. 

M.  How  can  things  be  set  on  fire  ? 

P.  You  put  other  burning  stuff  beside  the  coal  till  it 
begins  to  burn. 

M.  That  is  not  a  sufficient  answer.  What  happens 
to  the  coal  when  you  put  burning  stuffs  beside  it? 

P.  Now  I've  got  it.  The  coal  gets  warm,  and  so  catches 
fire. 

M.  That  is  right.  So  hot  coal  can  unite  with  oxygen 
and  cold  coal  can't.  And  that  is  the  reason  why 
coal  burns  in  the  fire,  and  not  in  the  cellar.  But  now 
I  will  tell  you  something.  It  happens  not  infrequently 
that  coal  which  is  left  lying  in  large  heaps  catches  fire 
of  its  own  accord  and  burns  without  anybody's  having 
lighted  it.  Such  a  heap  gets  hotter  and  hotter,  and 
when  it  is  not  cooled  by  spreading  it  out,  it  begins  to  burn. 

P.  I  can't  understand  why.  Where  does  the  heat  come 
from? 

M.  That  is  a  sensible  question.  It  comes  from  the 
burning  of  the  coal. 

P.  But  that  only  occurs  later  on. 

M.  No,  the  coal  is  always  burning.  Only  this  hap- 
pens so  slowly  at  low  temperatures  that  the  tempera- 
ture rises  very  slightly,  and  so  you  can  neither  see  it 
smoke  nor  catch  fire.  When  large  heaps  of  coal,  how- 
ever, lie  together,  so  as  to  prevent  heat  from  escaping, 
the  temperature  rises;  then  the  burning  takes  place  more 
quickly  and  the  temperature  rises  higher,  and  finally  rises 
so  high  that  the  coal  begins  to  glow  and  bursts  into  flame. 

P.  I  can't  imagine  coal  actually  burning  in  the  cellar. 

M.  I  will  remind  you  of  something  else.  Do  you 
remember  what  became  of  that  log  that  was  lying  out 
in  the  yard? 


126  CONVERSATIONS  ON  CHEMISTRY. 

P.  It  is  just  the  same  as  it  was. 

M.  No,  that  is  not  exactly  true.  If  wood  lies  for  a  long 
time  it  decays.     Do  you  know  what  that  means  ? 

P.  The  wood  gets  rotten  and  Hght. 

M.  Yes,  and  it  gets  smaller  and  smaller  and  finally 
disappears. 

P.  What  has  become  of  it? 

M.  It  has  been  burnt  too.  When  oxygen  is  kept  away 
from  wood,  it  doesn't  alter  Hke  that. 

P.  But  how  can  you  .call  it  burning  when  you  can't 
see  a  flame? 

M.  Burning  in  the  chemical  sense  of  the  word  is 
combination  with  oxygen,  whether  a  flame  is  visible  or 
not.  For  whether  a  flame  or  glowing  appears  depends 
upon  the  temperature  rising  high  enough,  at  least  to  500°; 
below  that  substances  don't  glow,  because  they  send  out 
no  light.  Whether  the  temperature  rises  so  high  doesn't 
depend  upon  the  chemical  change,  but  on  whether  the 
heat  is  sufficiently  kept  in. 

P.  Are  there  many  combustions  without  light  and 
heat? 

M,  Plenty.  But  without  evolution  of  heat,  such 
"flameless  combustions"  don't  take  place.  Just  as 
much  heat  is  evolved  as  if  combustion  had  taken  place 
with  a  flame.  When  a  chemical  change  takes  place, 
the  amount  of  heat  evolved  depends  on  the  beginning 
and  the  end;  it  doesn't  matter  if  it  takes  a  long  or  a 
short  time. 

P.  But  when  the  coal  on  the  fire  burns  brightly,  surely 
it  gets  hotter? 

M.  The  amount  of  heat  which  a  definite  quantity  of 
coal  gives  out  is  always  the  same.  But  of  course  if  you 
burn  more  coal  in  the  same  time,  the  fire  will  be  hotter. 


MORE  ABOUT  OXYGEN.  127 

P.  I  really  don't  quite  understand  that. 

M.  The  fire  gains  heat  by  the  burning  of  the  coal 
on  the  one  hand,  but  on  the  other  hand  it  loses  it  by 
heating  the  room.  It  is  something  like  pouring  water 
into  a  bucket  with  a  hole  at  the  bottom.  The  quicker 
the  water  runs  in  the  higher  it  will  stand  in  the  bucket. 
But  that  has  nothing  to  do  with  the  total  quantity  of 
water  that  you  pour  into  the  bucket. 

P.  Yes,  now  I  understand.  When  a  tree  decays,  it  is 
like  water  running  so  slowly  into  the  bucket  that  it's 
never  visible.  But  how  can  it  be  found  out  that  as  much 
heat  escapes  as  when  ordinary  burning  takes  place? 

M.  That  is  deduced  from  the  law,  that  energy  neither 
disappears  nor  is  created.  That  has  been  proved  and 
confirmed  in  innumerable  cases,  and  it  can  be  taken  for 
granted  in  cases  where  it  has  not  yet  been  proved. 

P.  But  it's  surely  possible  that  it  may  prove  to  be  false 
in  some  one  case. 

M.  Certainly.  But  then  other  things  would  show  it 
was  wrong,  and  the  error  would  soon  be  discovered. 
What  do  you  know  about  the  relations  of  animals  to  air? 

P.  They  can't  live  without  air,  so  I  always  make  holes 
in  the  lid  of  the  box  in  which  I  keep  my  silkworms. 

M.  But  there  is  air  in  the  box  anyhow,  along  with 
the  silkworms,  so  what  is  the  use  of  the  hole? 

P.  But  the  animals  require  fresh  air. 

M.  Why? 

P.  I  was  taught  that.  People  require  fresh  air  if  they 
are  to  keep  healthy. 

M,  Quite  right.  The  important  point  is  that  both  ani- 
mals and  men  shall  get  enough  oxygen.  Breathing  con- 
sists in  pumping  oxygen  into  the  lungs,  where  it  is  taken 
up  by  the  blood  and  led  through  all  parts  of  the  body. 


128  CONVERSATIONS  ON  CHEMISTRY. 

P.  What's  the  good  of  that? 

M.  To  burn  the  body. 

P.  You're  surely  in  fun? 

M.  No,  I'm  really  in  earnest.  The  process  of  the 
body  is  exactly  the  same  as  with  the  coal  in  the  cellar 
and  the  decaying  wood.  Certain  substances  in  the  body 
combine  with  oxygen,  although  not  so  quickly  as  with 
burning  wood. 

P.  Is  that  what  makes  the  body  warm? 

M.  Certainly.  A  dead  man  no  longer  breathes,  so 
his  body  gets  cold.  But  that  is  not  the  only  effect  pro- 
duced. The  body  does  all  sorts  of  work,  which  must 
be  produced  from  something,  because  work  can't  be 
made  of  nothing.  This  work  or  energy  is  produced 
from  its  combustion. 

P.  Then  surely  both  our  bodies  ought  to  have  burnt 
up  long  ago? 

M.  Quite  right.  If  we  were  not  always  introducing 
new  combustible  matter.  That  happens  when  we  take 
in  food. 

P.  Then  I  ought  to  be  able  to  eat  wood  and  coal. 

M.  Yes,  if  you  could  only  digest  them;  that  is,  if  your 
stomach  was  able  to  change  them  into  soluble  com- 
pounds, which  would  be  carried  with  the  juices  of  the 
body  to  all  the  parts,  where  they  could  combine  with 
oxygen.  For  that  matter,  cows  can  digest  wood  if  it 
is  given  them  sufficiently  fine.  The  substances  of  which 
grass  and  hay  are  made  are  not  very  different  from  wood. 

P.  Does  the  food  burn  in  the  lungs? 

M.  You  mean  because  air  enters  the  lungs  in  breath- 
ing? No,  oxygen  of  the  air  is  taken  up  by  the  blood  in 
the  lungs  and  passes  through  the  arteries  into .  all  the 
tissues  of  the  body;    and  there  it  meets  the    dissolved 


HYDROGEN.  129 

foods  and  bums  them  up.  Besides,  food  has  another 
use:  it  replaces  the  used-up  parts  of  the  body.  If  you 
think  of  your  body  as  a  steam-engine,  food  is  not  merely 
the  coal  which  makes  it  go,  but  also  the  metal  with 
which  it  is  repaired. 

P.  Is  that  the  case  with  all  animals,  or  only  with 
warm-blooded  ones? 

M.  You  think  that  cold-blooded  animals  don't  require 
it  because  they  are  not  warm?  That  isn't  right,  because 
they  are  all  a  httle  warmer  than  their  surroundings, 
and  they  all  breathe.  Ail  animals  require  food  and 
oxygen  because,  besides  keeping  themselves  warm,  they 
have  to  do  work.     They  move  about. 

P.  But  plants  don't  move.     What  about  them? 

M.  With  plants  there  is  a  different  state  of  affairs 
which  you  can't  quite  understand  yet.  We  will  come 
back  to  them,  and  then  you  will  see  these  things  in  a 
connected  manner. 

P.  It  has  been  a  jolly  lesson  to-day. 


17.  HYDROGEN. 

M.  We  will  talk  about  hydrogen  to-day.  What  do 
you  know  about  it? 

P.  It  comes  in  water. 

M.  That  is  not  expressed  well;  because  it  can  be 
obtained  from  water,  hydrogen  is  an  ingredient  of  water. 
What  other  ingredient  has  water  besides  that? 

P.  I  think  you  said  oxygen. 

M.  Right.  Water  consists  of  hydrogen  and  oxygen; 
that  is  to  say,  water  can  be  formed  from  both  these 
elements,  and  in  the  same  way  both  these  elements  can 


t^O  CONyERSATIONS  ON  CHEMISTRY. 

be  obtained  from  water.  How  do  you  think  oxygen 
could  be  made  from  water? 

P.  I  don't  quite  know.  Perhaps  water  could  be 
heated  and  it  might  decompose  into  the  two  elements, 
just  as  oxide  of  mercury  decomposes  into  its  ingredients. 

M.  That  is  quite  a  good  suggestion.  But  you  already 
know  what  comes  when  water  is  heated. 

P.  Yes,  steam. 

M.  Right.     Steam  is  only  water  in  another  form. 

P.  Perhaps  it  requires  greater  heat. 

M.  You  have  hit  upon  the  right  thing;  if  steam  is 
very  strongly  heated,  it  really  decomposes  into  oxygen 
and  hydrogen.  But  when  the  mixture  is  cooled,  it  com- 
bines again  *'to  form"  water,  and  one  can  only  tell  by 
a  special  artifice  that  it  has  been  decomposed.  Besides, 
only  a  mixture  of  oxygen  and  hydrogen  would  be  obtained, 
and  as  both  elements  are  gases,  it  would  not  be  easy  to 
separate  such  a  mixture. 

P.  Then  a  way  must  be  found  out  to  hold  fast  the  oxy- 
gen somehow.  Can't  it  be  made  Hquid,  like  the  mercury 
in  the  decomposition  of  mercuric  oxide? 

M.  Yes:  to  do  that  the  gas  mixture  must  be  cooled 
below  —  1 80°  C.  That  is  too  inconvenient  a  way.  I 
will  show  you  another:  we  do  not  separate  the  oxygen 
alone,  but  as  a  compound  with  some  other  element, 
and  arrange  it  so  that  the  compound  is  not  volatile. 

P.  I  don't  quite  understand. 

M,  I  will  tell  you.  We  pass  steam  over  glowing 
iron.  You  know  that  iron  combines  easily  with  oxy- 
gen. 

P.  Yes,  it  burns  with  a  lovely  rain  of  sparks. 

M.  Now  the  iron  acts  on  the  steam  in  such  a  way 
that  it  takes  the  oxygen   and    combines  to  form  iron 


HYDROGEN,  ^31 

oxide;  the  hydrogen  then  remains  over.  Iron  oxide  is 
a  soHd  substance  even  at  a  red  heat,  and  therefore 
remains  where  the  iron  was;  but  hydrogen  is  a  gas, 
and  passes  further  along;  it  can  then  be  collected  over 
water  in  the  same  way  as  oxygen. 

P.  That  still  seems  very  strange  to  me. 

M.  I  will  give  you  a  simile.  Oxygen  is  a  bone  which 
the  cat  hydrogen  had  to  start  with.  Then  the  dog  iron 
comes  along  and  takes  the  bone  from  the  cat,  and  the 
cat  hydrogen  must  run  away  without  the  bone. 

P.  Then  iron  is  stronger  than  hydrogen,  and  so  takes 
the  oxygen  away. 

M.  The  old  chemists  made  the  thing  out  to  be  some- 
thing like  that,  and  for  the  present  you  may  rest  content 
with  that  simile.  Later  on,  when  you  know  more  about 
chemistry,  you  shall  have  more  definite  examples. 

P.  May  I  see  the  experiment  ? 

M.  It  is  not  quite  simple  to  arrange,  for  a  fairly 
strong  heat  is  required.  The  best  way  is  to  fill  a  piece 
of  hard  glass  tubing  with  a  bundle  of  iron  gauze,  to 
heat  the  middle  till  it  glows,  and  lead  the  steam  over  it 
from  a  flask  in  which  water  is  boiling  at  the  other  end 
of  the  tube;  a  glass  tube  is  attached,  which  is  allowed 
to  discharge  into  an  inverted  flask  under  water.  Then, 
exactly  the  same  as  with  oxygen,  the  gas-bubbles  rise  and 
collect  in  the  flask. 

P.  What  a  shame  I  can't  see  it! 

M.  I  will  show  you  another  experiment  instead,  with 
which  you  can  see  much  the  same  kind  of  thing.  You 
remember  that  salt  contains  a  metallic  element  which 
is  called  sodium.  Here  is  some  of  this  metal.  I  have 
already  shown  you  (page  in)  that  it  combines  rapidly 
with  oxygen,  and  that  it  can  also  take  it  out  of  water. — 


132 


CONVERSATIONS  ON  CHEMISTRY, 


Now  I  take  a  little  piece  of  sodium  the  size  of  a  pea, 
wrap  it  up  in  a  piece  of  filter-paper,  and  stick  it  with  a 
pair  of  tongs  under  the  inverted  tube,  which  stands  in 
the  water  (Fig.  23). 


Fig  23. 

P.  The  sodium  is  slipping  out  of  the  paper!  Now  it 
seems  to  be  boiling,  and  some  air  has  collected  in  the 
tube. 

M.  Sodium  acts  in  the  same  way  as  I  told  you  iron 
did,  only  at  ordinary  temperature  and  much  more 
quickly.  It  takes  the  oxygen  of  the  water  and  sets  free 
tlie  hydrogen. 

P.  But  why  did  you  wrap  it  up  in  paper? 


HYDROGEN.  I33 

M.  Without  doing  that  it  would  have  been  difficult  to 
put  it  under  the  tube,  as  it  would  have  sprung  out  of  the 
tongs.  It  gets  heated  and  melts.  As  we  haven't  ob- 
tained a  great  deal  of  hydrogen,  I  will  repeat  the  experi- 
ment and  you  can  see  that  sodium  glides  about  on  the 
top  of  the  water  like  a  liquid  ball. 

P.  Why  didn't  you  take  more  sodium  at  once? 

M.  Because  the  experiment  is  not  quite  without  danger 
if  large  quantities  are  taken.  There  are  often  impurities 
in  the  sodium  which  make  it  explode,  so  that  only  small 
quantities  must  be  taken  in  order  that  an  explosion  may 
not  be  dangerous.  Remember  this  when  you  make  the 
experiment  alone. 

P.  Yes,  but  tell  me  what  has  become  of  the  compound 
of  sodium  and  oxygen  which  must  have  been  formed? 

M.  A  very  good  question!  Well,  as  it  is  neither  on 
the  top  of  the  water  nor  under  the  water,  where  can  it 
be? 

P.  In  the  water?    But  the  water  is  still  quite  clear. 

M.  Quite  right.  So  what  properties  must  the  com- 
pound have?  Think  of  our  first  talks  about  sugar  and 
copper  sulphate. 

P.  I  know!     It  has  dissolved. 

M.  Quite  right.  Taste  the  water  so  as  to  convince 
yourself. 

P.  Horrid,  like  soap! 

M,  You  have  discovered  one  reaction  of  the  compound 
which  is  formed.  But  we  will  speak  of  that  later.  Let 
us  pay  attention  to  hydrogen  at  present.  What  does  it 
look  like? 

P.  Like  air. 

M,  Yes,  hydrogen  is  a  colourless  gas.  Now  I  take 
the  tube  out  of  the  water,  closing  the  mouth  with  my 


134  CONVERSATIONS  ON  CHEMISTRY. 

thumb,  and  take  away  my  thumb  when  I  bring  it  near  a 
flame.     What  do  you  see? 

P.  The  hydrogen  appears  to  burn;  but  the  flame  is 
very  pale. 

M.  Quite  right.  Hydrogen  is  a  combustible  gas. 
But  in  order  to  learn  more  about  its  properties  we  should 
have  to  put  sodium  again  under  the  mouth  of  the  tube, 
and  that  would  be  tiresome.  I  will  rather  show  you 
another  method  of  making  hydrogen,  by  which  it  is 
much  easier  to  produce  large  quantities.  For  this  pur- 
pose we  take  other  compounds  of  hydrogen  which  give 
it  up  more  readily  than  water  does.  Such  a  compound 
is  hydrochloric  acid;  as  its  name  implies  it  consists  of 
hydrogen  and  chlorine. 

P.  Is  that  the  same  chlorine  that  is  contained  in  com- 
mon salt? 

M.  Certainly;  there  is  only  one  kind  of  chlorine. 
Here  is  a  solution  of  hydrochloric  acid  in  water,  as  it  is 
sold  by  the  druggists. 

P.  It  looks  just  like  water. 

M.  Yet  it  is  not  water.  I  pour  some  drops  into  a 
wine-glass,  and  fill  it  half  full  of  water;   taste  it. 

P.  Will  it  have  as  bad  a  taste  as  the  last  ? 

M.  No,  quite  different. 

P.  Yes,  it  tastes  sour.  But  not  very  pleasant,  and  it 
makes  my  teeth  rough. 

M.  Yes,  because  it  tastes  acid  it  is  called  an  acid. 

P.  Why  did  you  pour  in  so  much  water? 

M.  Because  strong  hydrochloric  acid  is  poisonous, 
though  dilute  acid  isn't.  The  reason  your  teeth  felt  rough 
was  that  the  acid  attacks  the  substance  of  which  the 
teeth  are  made.  But  now  we  will  begin  our  experiment. 
I  have  here,  in  a  flask,  clippings  of  sheet  zinc.     Now  we  11 


HYDROGEN. 


135 


put  into  the  flask  a  cork  provided  with  two  holes. 
Through  one  passes  a  tube  with  a  funnel  at  the  top, 
and  it  reaches  to  the  bottom  of  the  flask;  and  through 
the  other  a  short  bent  glass  tube  to  which  I  connect  my 
delivery-tube  with  a  piece  of  rubber  tubing — the  one  I 
used  before  for  oxygen  (Fig.  24).     Now  I  pour  hydro- 


FlG.  24. 

chloric  acid  through  the  funnel,  and  at  once  you  see 
gas  coming  off. 

P.  Quick!    Place  the  flask  over  it  to  catch  it. 

M.  No,  I  will  first  collect  some  gas  in  a  test-tube. 
Stop,  here  is  the  first  one  full.  I  Kft  it  out  and  hold  it 
to  the  flame.     What  happens? 

P.  Nothing.  It  must  have  been  the  air  that  was  in 
the  flask. 

M.  Quite  right.     Now  I  repeat  the  experiment. 

P.  That  gave  a  loud  crack. 

M.  I  will  collect  some  more  samples.     You  see,  the 


136  COhiyERS/iTIONS  ON  CHEMISTRY. 

first  ones  explode,  but  now  the  gas  bums  quite  quietly, 
like  the  hydrogen  we  made  by  using  sodium.  Now  we 
can  collect  it  in  flasks,  and  when  the  evolution  of  gas 
slackens,  we  only  need  to  add  a  little  more  hydrochloric 
acid  and  it  begins  again. 

P.  Please  explain  all  this  to  me. 

M.  With  pleasure;  First  the  production  of  hydrogen 
from  hydrochloric  acid  and  zinc;  that  is  just  like  the 
formation  of  hydrogen  from  water  and  iron.  The 
chlorine  takes  the  zinc  rather  than  stay  with  the  hydrogen, 
and  so  the  hydrogen  is  set  free.  It  is  very  convenient 
that  this  takes  place  at  the  ordinary  temperature,  and  with- 
out the  necessity  of  using  a  dangerous  metal  like  sodium. 

P.  I  understand  that.     But  what  niade  it  pop  ? 

M.  Look,  here  I  have  a  test-tube  that  is  only  half 
filled  with  water.  I  close  it  with  my  thumb  and  place 
its  mouth  under  water.  Now  the  tube  is  half  full  of  air. 
I  displace  the  water  from  the  other  half  of  the  tube  with 
hydrogen,  which  is  not  explosive.  If  I  bring  this  tube 
with  a  mixture  of  hydrogen  and  air  near  the  flame — 

P.  By  jove!    what  a  thundering  crack! 

M.  You  see  that  a  mixture  of  air  and  hydrogen  ex- 
plodes, though  pure  hydrogen  doesn't.  If  I  were  to 
light  such  a  mixture  in  a  flask,  it  would  burst,  and  the 
pieces  might  cause  serious  injury.  Now,  as  there  was 
air  originally  in  the  bottle,  it  would  have  made  a  dangerous 
mixture;  and  it  was  only  after  the  air  had  been  driven 
out  by  hydrogen  that  pure  hydrogen  escaped.  Remember 
that  you  must  always  test  the  gas  when  you  make  hydrogen 
in  this  way,  and  not  collect  the  gas  before  it  burns  quietly. 

P.  So  the  explosion  is  a  test  for  air  in  the  hydrogen? 
But  why  did  it  pop? 

M.  Because  the  hydrogen  was  completely  mixed  with 


HYDROGEN.  137 

the  oxygen,  which  it  required  for  burning,  and  the  flame, 
when  it  once  starts,  spreads  immediately  through  the 
whole  mass.  But  when  pure  hydrogen  burns  in  the 
air,  combination  can  take  place  only  when  the  two 
gases  touch.  The  shape  of  the  surface  when  this 
takes  place  is  the  same  as  that  of  the  flame.  Can  you 
tell  me  why  a  quietly  burning  flame  like  that  of  a  candle 
has  a  conical  shape? 

P.  Let  me  think.  Yes,  the  burning  gas  rises  and 
burns,  and  because  it  grows  less  the  flame  gets  narrower. 

M.  Quite  right.  Now  let  us  go  back  to  hydrogen. 
I  fill  two  tubes  with  it,  and  leave  one  with  its  mouth  up 
and  the  other  with  its  mouth  down.  Which  will  the 
hydrogen  stay  in? 

P.  When  you  ask  questions,  I  am  afraid  of  some  catch, 
and  am  likely  to  answer  wrongly.  So  I'll  say  the  oppo- 
site of  what  I  think.  The  hydrogen  will  stay  in  the  tube 
with  the  mouth  downwards. 

M.  Let  us  try.  First  I  bring  into  the  flame  the  tube 
which  has  its  mouth  upwards,  and  try  to  set  its  contents 
on  fire;  nothing  happens,  and  when  I  hold  a  burning  match 
in  it,  it  goes  on  burning;  and  so  the  tube  contains  air. 
Now  the  other  tube.     I  hold  its  mouth  above  the  flame — 

P.  I  was  right  after  all.  The  hydrogen  stayed  in 
it.     It  burns  with  a  pop.     That  is  most  astonishing. 

M.  Now  think.  What  did  I  tell  you  about  the  density 
of  hydrogen? 

P.  That  it  was  the  lightest  of  all  substances.  But 
still  it  has  some  weight  and  ought  to  fall.  Oh,  now  I 
know.  It  is  lighter  than  air,  and  so  it  floats  up  in  the 
air  like  a  cork  in  water.  But  in  an  empty  space  it  ought 
to  fall. 

M.  So  it  would  if  it  were  a  solid  or  liquid.     But  a 


1 3^  CON  VERSA  TIONS  ON  CHEMIS  TR  Y. 

gas  spreads  all  through  an  empty  space  till  it  fills  it 
equally  throughout.  Now  do  you  understand  the  experi- 
ment? 

P.  Yes;  the  hydrogen  tries  to  rise  in  the  air,  and  if  it 
can  find  an  opening  above,  it  escapes ;  but  if  the  opening 
is  below  it,  it  must  stay  there. 

M.  Quite  right.  Now  you  deserve  a  treat,  and  I 
will  show  you  a  pretty  experiment  which  will  illustrate 
its  behaviour  even  better.  I  have  made  some  soap-suds. 
Now,  by  means  of  a  piece  of  rubber,  I  pin  to  the  gas 
dehvery-tube  a  piece  of  glass  tubing  stopped  loosely  with 
cotton-wool,  and  plunge  the  end  below  the  soap-suds. 

P.   You  can  really  blow  soap-bubbles  with  hydrogen? 

M.  Yes,  and  here  is  a  very  big  one;  it  separates  from 
the  soap-suds,  and  rises  like  a  balloon. 

P.  Oh,  how  jolly!  But  what  is  the  use  of  the  cotton- 
wool in  the  tube  ? 

M.  The  hydrogen  carries  innumerable  little  drops 
of  acid  with  it  as  a  sort  of  mist,  and  when  these 
touch  the  soap-bubble,  it  bursts.  But  the  little  drops 
stick  in  the  cotton-wool  and  don't  get  into  the 
bubble. 

P.  Are  the  big  balloons  that  are  sold  in  shops  filled 
with  hydrogen? 

M.  Yes. 

P.  I  used  to  have  one,  and  the  first  day  it  went  up 
all  right,  on  the  second  it  would  hardly  rise,  and  the 
third  day  it  wouldn't  rise  at  all.  Did  the  hydrogen 
grow  heavier? 

M.  No,  but  hydrogen  is  such  a  fine-grained  stuff  that 
it  can't  be  kept  in  by  a  thin  sheet  of  india-rubber;  it 
passes  out,  and  some  air  enters  instead. 

P,  Oh  yes,  I  remember  my  balloon  got  much  smaller. 


OXYGEU  AND  HYDROGEN.  139 

I  thought  at  first   that  the  mouth  hadn't  been  tightly 
enough  tied,  but  it  was. 

M.  Quite  right.  You  see  you  shouldn't  keep  hydrogen 
in  any  kind  of  vessel  for  a  very  long  time;  it  generally 
gets  out  and  air  enters,  making  an  explosive  mixture. 


18.  OXYGEN  AND  HYDROGEN. 

M,  What  did  you  learn  yesterday  about  hydrogen? 

P.  That  it  can  be  made  from  its  compounds  by  taking 
away  by  means  of  another  substance  what  it  is  com- 
bined with.  It  can  be  set  free  from  water,  in  which  it 
is  combined  with  oxygen,  by  iron  or  sodium. 

M.  And  how  do  the  two  metals  dififer  in  doing  it? 

P.  Iron  does  it  only  when  glowing,  sodium  does  it  at 
the  ordinary  temperature. 

M.  And  further? 

P.  You  can  take  hydrochloric  acid  and  zinc.  The 
zinc  takes  the  chlorine,  and  the  hydrogen  comes  out. 

M.  What  properties  has  hydrogen? 

P.  It  looks  colourless,  like  air,  but  it  weighs  much  less. 
But  you  never  told  me  how  much  hghter  it  was  than  air. 

M.  Its  density  is  about  14  times  less  than  that  of  air. 
One  litre  of  hydrogen,  like  what  we  have  in  the  flask, 
weighs  less  than  Yn  gram.  What  more  do  you  know 
about  hydrogen? 

P.  It  burns  in  air,  and  if  it  is  first  mixed  with  air,  it 
gives  a  loud  bang,  because  the  whole  mass  burns  suddenly. 

M.  Quite  right.  What  is  made  from  hydrogen  when 
it  burns? 

P.  You  never  told  me. 

M.  You  ought  to  have  been  able  to  discover  it  for 
yourself.    Just  think  a  minute.   What  happens  on  burning  ? 


140 


CONVERSATIONS  ON  CHEMISTRY. 


P.  The  substances  combine  with  the  oxygen  of  the  air. 

M.  Right.  Now  if  hydrogen  combines  with  oxygen, 
what  is  made?  Don't  you  remember  that  we  have  just 
been  speaking  about  such  a  compound;  which  was  it? 

P.  You  told  me  about  water.     Should  water  be  made  ? 

M.  Certainly,  water  is  made.  We  soon  see  it.  Don't 
you  remember  how  I  showed  you  the  formation  of  water 
with  a  burning  candle? 

P.  Yes,  with  a  large  beaker  that  was  held  over  it. 
It  became  covered  with  drops  of  water. 

M.  We  can  do  that,  too,  with  a  hydrogen  flame.  I 
fasten,  to  the  apparatus,  a  glass  tube  the  end  of  which 
I  have  made  narrower,  and  let  the  hydrogen  burn  from 
it  (Fig.  25).    There,  you  see  the  drops  at  once. 


Fig.  25. 

P.  How  do  you  make  a  point  like  that? 

M.  You  hold  the  tube  in  the  flame,  turning  it  till 
the  place  is  quite  soft,  then  you  pull  it  apart  length- 
ways, and  cut  through  the  narrow  part  with  the  glass- 
cutter. 


OXYGEN  AND  HYDROGEN. 


141 


P.  Please  let  me  do  it.  Now  the  tube  is  soft,  and 
now  I  pull  it.     Oh,  it  is  as  thin  as  a  hair! 

M.  You  pulled  too  hard  and  too  quickly.  Besides, 
this  thin  hair  is  also  a  tube,  as  glass  doesn't  fall  together 
with  pulhng. 

P.  Really?  I  can  hardly  believe  that  there  can 
be  such  a  thin  tube. 

M,  Break  a  piece  o£f  and  put  the  end  in  the  ink  and 


Fig.  26. 


then  you  can  see  how  the  black  liquid  comes  through. 
But  we  must  return  to  our  hydrogen.  Hydrogen  can 
combine  not  only  with  free  oxygen,  but  can  take  oxygen 
from  other  compounds.  Do  you  remember  mercuric 
oxide?    What  sort  of  substance  was  that? 

P,  A  red  powder;  a  compound  of  mercury  and  oxygen. 

M,  Yes.    I  take  a  little  mercuric  oxide,  put  it  in  a 


1 4  2  CONyERSA  TIONS  ON  CHE  MIS  TR  Y. 

glass  tube  which  I  attach  to  the  hydrogen  apparatus, 
let  the  hydrogen  pass  over  it,  and  heat  it  carefully  (Fig.  26). 

P.  Mercury  separates  again. 

M.  Right,  but  further? 

P.  There  are  clear  drops  that  look  like  water;  is  it 
water? 

M.  Yes.  This  time  the  hydrogen  has  taken  away  the 
oxygen  from  the  mercuric  oxide  to  form  water,  and  the 
mercury  is  set  free. 

P.  Does  that  happen  with  all  oxygen  compounds? 

M.  Not  with  all,  but  with  a  great  many.  Most 
oxides  of  the  heavy  metals  can  be  thus  changed  into 
metals.  This  change  is  called  reduction,  the  opposite 
of  oxidation.  The  changing  of  a  metal  into  its  oxide 
is  called  oxidation,  the  changing  of  an  oxide  to  the  metal, 
a  reduction.  As  hydrogen  makes  this  change  possible 
it  is  called  a  reducing  agent.     Notice  this  name. 

P.  I  have  learned  a  great  deal  that  I  didn't  know 
before. 

M.  I  will  make  it  easier  for  you  by  showing  you  some 
more  experiments.  This  black  powder  is  called  copper 
oxide.  It  is  easily  formed  if  copper  is  heated  for  some 
time  in  the  air.  I  put  some  of  it  in  a  tube,  pass  hydrogen 
over  it,  and  heat  it  again;  do  you  see  what  the  copper 
looks  like? 

P.  Yes,  the  powder  is  getting  red  like  copper,  and 
again  drops  of  water  are  falling  in  the  tube. 

M.  I  take  away  the  flame  and  let  it  get  cold,  while  the 
hydrogen  is  going  through.  Now  I  can  shake  out 
the  red  grains,  and  if  I  rub  them  in  the  mortar  you  will 
see  they  will  shine  like  metal. 

P.  How  pretty!  Why  do  they  only  shine  after  they 
are  rubbed? 

M.  The   copper  was  not   even  and  smooth  before. 


OXYGEN  AND  HYDROGEN.  143 

As  the  oxygen  has  separated  from  the  copper  oxide, 
the  copper  remains  behind  Hke  a  sponge. — ^This  yellow 
powder  is  called  oxide  of  lead  and  is  a  compound — • 

P.  Of  lead  and  oxygen. 

M.  That  is  a  good  answer.  I'll  allow  you  to  reduce 
it  yourself  for  that.     Do  it  in  the  same  way  as  before. 

P.  Bright  drops  like  mercury  have  appeared;  is  that 
lead? 

M.  Yes;  as  lead  melts  very  easily,  it  is  obtained  at 
once  in  a  liquid  form.  Pour  these  drops  onto  a  piece  of 
paper  and  then  you  can  see  how  they  solidify  into  a 
soft  and  unelastic  metal  which  is  easily  bent.  Those 
are  the  properties  of  lead.  But  now  we  are  going  to 
do  a  special  experiment.  This  is  the  iron  oxide  which 
we  obtained  before  by  burning  iron  powder  in  the  air. 
We  are  going  to  reduce  this  by  means  of  hydrogen. 

P.  How  can  that  happen?  You  told  me  yourself 
yesterday  that  iron  is  stronger  than  hydrogen,  because 
it  takes  the  oxygen  out  of  water  and  drives  away  hy- 
drogen. So  how  can  hydrogen  become  stronger  than 
iron? 

M.  One  must  make  experiments  even  when  one 
thinks  they  won't  come  to  anything.  For  every  con- 
clusion we  draw  may  be  erroneous,  and  must  be  tested 
by  experiment. 

P.  I  am  really  curious  about  it.  Do  you  see,  nothing 
is  happening;  the  broken  bits  only  become  a  little  blacker. 

M.  Just  notice  carefully  the  further  part  of  the  tube. 

P.  H'm!  There  really  appear  to  be  drops  of  water 
coming  there.  On  the  one  hand,  it  looks  as  if  nothing 
were  happening,  and  on  the  other  hand,  as  if  something 
were  happening  after  all. 

M.  I  will  let  it  cool  again  while  the  hydrogen  is  still 


144  CONVERSATIONS  ON  CHEMISTRY. 

passing  over  it.     Now  just  rub  the  black  mass  in  the 
mortar,  as  we  did  with  copper. 

P.  It  is  becoming  bright  too 

M.  Then  it  is  metaUic  iron. 

P.  Now  please  tell  me  how  it  is  possible  that  there 
can  be  such  a  contradiction.  I  thought  that  laws  of 
nature  always  held. 

M.  What  law  of  nature  has  changed  here  ? 

P.  One  force  cannot  well  be  greater  and  smaller 
than  the  other.  First  iron  was  stronger  than  hydrogen, 
and  afterwards  hydrogen  was  stronger  than  iron.  That 
is  surely  a  contradiction. 

M.  The  contradiction  only  lies  in  this,  that  you  look 
upon  the  reason  of  chemical  change  as  a  mechanical 
force;  a  force  Hke  this  doesn't  let  itself  be  known  or 
measured  beforehand. 

P.  What  is  it,  then? 

M.  If  I  were  to  answer  this  question,  you  wouldn't 
understand  me.  You  must  know  many  facts  about 
chemistry  before  you  can  think  of  connecting  them  by  a 
theory. 

P.  But  can't  you  say  something  that  would  put  me  on 
the  right  track? 

M.  Yes;  out  of  your  own  wrong  example;  one  man 
can  carry  a  certain  amount  of  water;  but  if  much  more 
water  comes  it  will  carry  the  man  away. 

P.  So  you  mean  that  in  chemical  changes  it  depends 
on  which  substance  is  present  in  the  greatest  quantity. 

M.  Something  like  that.  But  we  must  go  back  to  our 
hydrogen.  You  know  now  that  in  the  combining  of 
hydrogen  with  oxygen  water  is  formed,  and  that  for 
this  purpose  oxygen  can  be  taken  out  of  other  compounds. 
But  there  is  still  something  else  that  happens:   I  set  my 


OXYGEN  AND  HYDROGEN,  145 

hydrogen  apparatus  going  again,  and  after  the  mixed 
gas  has  gone,  light  the  hydrogen.  You  see  that  the 
flame  is  fairly  pale. 

P.  At  first  it  is  always  bluish,  but  afterwards  it 
becomes  lighter,  and  looks  yellow. 

M.  That  is  because  the  glass  tube  from  which  the 
hydrogen  bums  becomes  hot.  The  element  sodium 
is  contained  in  glass,  as  you  already  know.  A  Httle 
evaporates  from  the  hot  glass,  and  this  vapour  colours  the 
flame  yellow. 

P.  How  is  that? 

M.  Glowing  sodium  sends  out  a  yellow  light,  just  as, 
for  example,  the  metal  copper  reflects  red  light.  The 
yellow  colouring  of  the  flame  is  a  test  for  sodium;  it  is 
always  to  be  found  when  sodium  is  present,  and  is  absent 
when  there  is  none  there. 

P.  But  nearly  all  flames  are  yellow. 

M.  In  nearly  all  burning  materials  sodium  is  present, 
and  a  very  little  is  enough  to  make  the  yellow  colour. 
But  we  can  make  a  pure-coloured  hydrogen  flame.  I 
have  here  a  little  piece  of 
platinum-foil;  I  make  it 
soft  by  heating,  and  then 
wrap  it  firmly  round  a 
knitting-needle;  so  I  get 
a    very     serviceable    little      A 

tube   of  platinum.     I   put      ■  ^ 

a  few  millimetres  of  this  in 
a  glass  tube  which  is  slightly  wider  than  it,  and  heat  the 
place.  You  see  how  the  glass  tube  lies  round  the  plati- 
num ?  Now  it  is  melted  and  closed  up  all  round,  and  I 
have  a  burner-tube  with  a  platinum  tip  which  I  can 
afterwards  bend  to  a  right  angle.    (Fig.  27.) 


r' 


U6  CONVERSATIONS  ON  CHEMISTRY. 

P.  Why  platinum? 

M.  Because  this  metal  is  only  melted  with  great  diffi- 
culty and  is  not  easily  attacked.  When  I  attach  the 
tube  to  the  hydrogen  apparatus,  I  can  leave  the  gas 
burning  for  hours,  and  the  flame  will  never  become 
yellow.  Now  I  hold  a  morsel  of  platinum  wire  in  the 
hydrogen  flame.     What  do  you  see? 

P.  The  wire  shines  very  brightly;  the  flame  seems 
to  be  very  hot,  then. 

M.  Quite  right.  A  glowing  body  glows  more  brightly 
the  hotter  it  is.  With  gases  this  is  not  the  case:  glowing 
hydrogen  vapour  gives  very  little  light;  that  is  why  the 
hydrogen  flame  is  so  faint,  while  it  makes  every  solid 
body  that  it  reaches  glow  so  brightly. 

P.  Every  one? 

M.  Every  one  that  doesn't  melt  or  turn  to  vapour. 
Here  I  have  a  fragment  of  incandescent  mantle.  Just 
look  how  brightly  it  glows.  And  iron  wire  begins  to 
glow  brightly  at  first  too,  but  it  soon  melts  and  burns. 
Very  well,  then,  tell  me  what  is  formed  in  the  flame 
besides  water? 

P.  Heat. 

M.  Right.  What  is  heat  ?  Remember  what  we  spoke 
about  a  short  time  ago,  when  we  were  talking  about 
combustion. 

P.  Yes,  you  had  a  special  name  for  it :  I  think  energy. 

M.  Quite  right.     What  is  energy? 

P.  Everything  that  comes  from  work  and  can  be 
changed  into  work.  How  can  you  get  work  from  burn- 
ing hydrogen? 

M.  Now  you  heard  for  yourself  what  a  loud  bang  a 
mixture  of  hydrogen  and  air  made,  and  I  told  you  also 
that  it  could  break  glass.     Work  is  used  up  for  that. 


OXYGEN  AND  HYDROGEN.  147 

P.  A  funny  sort  of  work.  Mother  would  very  soon 
put  a  stop  to  it  if  I  wanted  to  break  her  glasses  and  said 
I  was  at  work. 

M.  It  is  work  all  the  same,  as  it  requires  a  certain 
amount  of  exertion.  To  be  sure,  it  is  not  useful  work. 
But  when  the  miller  grinds  his  corn,  his  mill  does  similar 
work,  and  that  is  useful. 

P.  Can  any  useful  work  be  done  with  the  explosive 
gas? 

M.  Certainly.  There  is  a  certain  sort  of  machine 
in  which  an  explosive  mixture  of  air  and  coal-gas  is 
burnt.  The  explosion  drives  a  piston  forward,  and  as  the 
machine  turns  further,  gas  and  air  are  again  sucked  in 
to  form  the  explosive  mixture,  and  this  is  again  exploded, 
so  that  the  piston  each  time  receives  a  powerful  push. 
Such  gas-engines  are  now  made  of  the  largest  dimen- 
sions, and  in  many  respects  are  much  better  than  steam- 
engines. 

P.  Are  engines  of  motors  made  like  that?  They  puff 
in  the  same  way. 

M.  They  are  something  like,  only  with  them  the  explo- 
sive gas  is  made  with  benzine  vapour. 

P.  Then  explosive  gas  can  be  made  with  all  sorts  of 
things  ? 

M.  li  di  combustible  gas  or  vapour  is  mixed  with  as 
much  air  or  oxygen  as  is  necessary  to  burn  them  up,  an 
explosive  gas  is  always  obtained.  For  then  the  flame 
can  always  go  through  the  whole  mass  and  burn  it  at 
once,  whereas  otherwise  the  burning  can  only  take  place 
where  the  air  **  reaches." 

P.  Yes,  you  made  that  clear  before. 

M.  I  made  something  else  clear  to  you  too.  How 
can  the  hydrogen  flame  be  made  still  hotter  than  it  is  at 


148  CONyERSATlONS  ON  CHEMISTRY. 

present?  Do  you  remember  what  I  told  you  about 
burning  in  air  and  in  pure  oxygen? 

P.  Yes,  I  know:  if  you  were  to  burn  hydrogen  with 
pure  oxygen,  the  nitrogen  in  the  air  wouldn't  need  to 
be  heated  with  it  and  the  flame  would  be  hotter. 

M.  Right.    How  would  you  do  that? 

P.  I  would  let  the  hydrogen  flame  burn  in  a  flask 
which  contained  oxygen. 

M.  Quite  right,  but  not  convenient.  A  very  high 
temperature  is  obtained  if  oxygen  is  blown  into  the  hydro- 
gen flame. 

P.  But  how  can  that  be  done? 

ikf.  Well,  we  could  take  an  empty  india-rubber  balloon 
and  fill  it  with  oxygen  and  then  press  it ;  then  the  oxygen 
would  stream  out  of  the  opening.  But  I  will  show  you 
how  a  proper  gasometer  is  made.  I  have  here  two 
very  large  flasks  which  are  provided  with  corks,  in  each 
of  which  are  two  holes.  Through  one  hole  a  siphon 
of  glass  goes  to  the  bottom,  through  the  other,  a  short 
bent  tube  (Fig.  28).  Both  siphons  are  connected  by  a 
piece  of  rubber  tubing,  and  one  flask  is  filled  with  water. 

P.  I  can't  quite  see  what  use  all  this  is  going  to  be. 

M.  Just  notice:  I  now  connect  an  oxygen  apparatus 
(page  84)  to  the  bent  tube  of  the  flask  filled  with  water, 
and  place  the  other  flask  at  a  lower  level.  If  I  make 
oxygen  by  heating,  it  goes  into  the  upper  flask  and  the 
water  runs  through  the  india-rubber  tube  into  the  lower 
one. 

P,  That  is  pretty. 

M.  So  now  the  higher  flask  is  full  of  oxygen.  I  take 
the  oxygen  apparatus  off,  and  close  the  rubber  tube  with 
a  clip. 

P.  What  is  a  clip? 


OXYGEN  AND  HYDROGEN. 


149 


M.  It  is  a  wire  spring  which  squeezes  the  rubber 
so  as  to  close  it.  Such  a  chp  is  very  easy  to  put  on,  and 
often  closes  a  tube  better  than  a  stop-cock,  so  that  it 
is  very  often  used  in  chemistry. 

P.  I  like  that,  it  is  so  simple  and  useful. 

M.  Now  we  can  let  our  oxygen  stream  out  whenever 
we  wish.     I  only  require  to  raise  the  flask  containing  the 


Fig.  28. 

water,  and  the  oxygen,  under  the  pressure  of  the  water 
from  such  a  height,  streams  out  if  I  open  the  cHp.  If 
I  close  the  cHp,  the  stream  stops  again.  If  I  don 't  want 
oxygen  for  some  time  I  put  the  higher  flask  low  again 
and  there  is  no  more  pressure. 

P.  I  Hke  that. 

M.  Now  I  fix  my  glass  tube  with  the  platinum-point 
on  the  gasometer  and  fasten  it  so  that  the  point  pro- 
jects into  the  flame  of  the  spirit-lamp.     I  let  the  oxygen 


ISO  CO^yERSATIONS  ON  CHEMISTRY. 

pour  out,  so  that  the  flame  will  be  blown  to  the  side;  at 
the  same  time  it  will  be  small  and  pointed  and  very  hot. 

P.  It  only  looks  a  little  brighter. 

M.  I  hold  a  thin  piece  of  platinum  wire  in  it;  you 
see  that  it  not  only  glows  white-hot,  but  soon  melts. 
Now  a  pretty  round  ball  is  formed  at  the  end  of  the 
wire,  and  if  I  heat  it  longer  it  will  fall  off. 

P.  It  is  so  bright  that  one  can  hardly  see.  But  you 
were  going  to  show  me  the  temperature  of  the  hydrogen 
flame. 

M.  In  this  flame  it  is  the  hydrogen  of  which  the  spirit 
mainly  consists  which  is  burnt.  But,  to  produce  a  good 
hydrogen  flame,  we  must  make  our  apparatus  somewhat 
larger  and  more  powerful.  .  As  it  is  at  present,  it  gives 
out  a  lot  of  gas  if  fresh  acid  has  been  put  in,  but  soon  less, 
and  a  regular  flame  cannot  be  obtained.  We  will  make 
an  apparatus  that  will  give  us  just  as  much  or  as  Httle 
gas  as  we  need. 

P.  I  am  curious  to  see  how  you  will  make  that  appa- 
ratus. 

M.  I  take  two  flasks  with  the  right  corks  and  tubes, 
exactly  the  same  as  the  oxygen-gasometer,  only  that  I 
take  rather  smaller  flasks.  One  of  them  is  filled  with 
zinc  and  in  the  other  is  dilute  hydrochloric  acid;  the 
latter  is  raised  higher  than  the  former.  When  I  open 
the  cHp  which  is  on  the  zinc  flask,  the  hydrochloric  acid 
comes  through  to  the  zinc,  and  hydrogen  is  evolved. 

P.  But  nothing  comes. 

M.  The  siphon  is  not  filled  yet,  and  so  cannot  work. 
But  I  only  need  to  blow  down  the  tube  of  the  hydrochloric- 
acid  flask.     Now  it  comes. 

P.  Yes,  now  the  acid  is  bubbling.  But  why  did  you 
first  put  a  layer  of  pebbles  in  the  zinc  flask? 


OXYGEN  AND  HYDROGEN.  l^^ 

M.  That  you  will  soon  see.  I  close  the  clip  which 
lets  the  hydrogen  out.     What  do  you  see? 

P.  The  acid  goes  back  again  through  the  siphon  into 
the  upper  flask.  Ah,  now  I  understand.  The  hydrogen 
which  can't  come  out  any  more  presses  the  acii  out  of 
the  lower  flask  into  the  upper  one. 

M.  Quite  right.  However,  as  not  all  the  acid  can 
have  gone  back,  because  the  bottom  is  uneven,  some 
must  have  stayed  behind,  which  would  work  further  on 
the  zinc.  But  now  this  residue  merely  remains  with  the 
pebbles. 

P.  That  is  pretty:    a  regular  automatic  machine. 

M.  I  first  test,  my  hydrogen  to  see  if  it  is  pure,  and 
then  light  it.  I  open  the  clip  so  as  to  give  a  fairly  large 
flame.  For  this  purpose  the  cHp  is  provided  with  a 
screw  (Fig.  29).     Now  I  bring  up  the  platinum  tip  with 


Fig,  29. 

the  oxygen,  and  you  see  how  small  and  pointed  the 
flame  becomes.  A  piece  of  platinum  wire  melts  far  more 
easily  than  before.  A  steel  watch-spring  that  is  heated 
at  the  end,  first  glows  white-hot,  and  then  burns  with 
lovely  streaming  sparks,  as  in  oxygen.  A  piece  of  chalk 
that  I  have  pointed  begins  to  glov/,  and  gives  such  a 
brisfht  white  light  that  it  looks  like  sunshine. 

p.  That  is  a  pretty  firework! 

M.  It  shows  you  that  the  flame  of  pure  hydrogen  and 
oxygen,  or,  to  put  it  shortly,  the  oxyhydrogen  blowpipe, 
is  really  uncommonly  hot. 


152  CONVERSATIONS  ON  CHEMISTRY. 

P.  That  must  be  about  the  highest  temperature  that 
can  be  reached? 

M.  No;  the  flame  is  only  2000°  C,  while  between  the 
charcoal- points  of  an  electric-arc  lamp  over  3000°  C.  is 
reached.  But  still  it  is  a  very  high  temperature,  which 
our  furnaces  never  nearly  attain. 

P.  What  a  lot  I  have  seen  and  learned  to  day! 


19.  WATER. 

M.  To-day  we  shall  study  water  itself,  after  having 
learnt  about  its  constituents  and  formation.  You  know 
that  water  occupies  the  greater  part  of  the  earth's  surface. 

P,  Yes,  about  five  sevenths. 

M.  Now,  the  water  which  forms  the  oceans,  lakes, 
and  rivers  is  not  by  any  means  pure  water,  but  contains 
many  other  dissolved  substances. 

P.  I  know  that  sea-water  contains  salt;  but  I  don't 
know  anything  about  other  water,  or  that  other  sub- 
stances should  be  in  it. 

M.  How  do  you  know  of  the  presence  of  salt  in  sea- 
water? 

P.  By  its  salt  taste. 

M.  Quite  right.  Then  do  all  other  waters  taste  the 
same,  rain-water  and  spring- water,  for  example? 

P.  No.    I  once  tasted  rain-water;  it  had  a  horrid  taste. 

M.  Well,  you  must  conclude  from  the  difference  of  taste 
of  these  other  waters  that  they  contain  different  substances. 
Here  you  have  a  specimen  of  pure  water;  just  taste  it. 

P.  It  tastes  just  as  nasty  as  rain-water.  How  is  pure 
water  ma.de? 

M.  By  distillation.     That  is  to  say,  it  is  first  changed 


HEATER. 


153 


into  steam,  and  this  steam  is  cooled  again  till  it  changes 
into  liquid  water. 

P.  But  how  is  the  water  purer  for  that? 

M.  The  impurities  which  are  contained  in  ordinary- 
water  do  not  change  into  steam,  as  they  are  not  volatile. 
I  take  some  ordinary  drinking  water  and  add  ink  to  it,  so 
that  you  can  see  the  impurity  quite  distinctly;  if  I  distil 
this  black  liquid,  a  pure  and  clear  water  comes  over. 

P.  I  should  like  t3  see  that.     How  is  it  done? 

M.  In  different  ways.  We  will  do  it  in  the  simplest  way 
first.  I  put  a  cork  with  a  hole  in  it  into  this  thin-walled 
flask,  and  by  this  means  attach  to  the  flask  a  tube  which 
is  rather  sharply  bent  over.  I  pour  my  black  water  into 
the  flask,  and  heat  it  till  it  boils  (Fig.  30). 


Fig.  30. 

P.  Now  there  is  some  steam  in  the  tube,  and  now  a 
drop  of  water  is  running  down ;   it  is  really  quite  clear. 

M.  We  will  put  another  flask  over  the  lower  end  of 
the  tube,  to  collect  our  distilled  water. 

P.  Now  this  flask  is  being  covered  up  inside  with  mist 
and  now  the  steam  is  coming  out,  and  isn't  condensing. 

M.  What  is  the  reason  of  that? 

P.  The  flask  has  become  too  hot,  and  can't  cool  the 
steam  any  more. 


154  CONyERSATIONS  ON  CHEMISTRY. 

M.  Quite  right.  To  distil  properly,  we  must  provide 
a  cooler.  I  can  do  that  simply:  I  place  a  dish  of  cold 
water  so  that  the  flask  stands  in  it;  that  will  keep  it 
cool. 

P.  But  if  the  water  gets  warm  ? 

M.  Then  we  must  stop.  You  have  here  an  important 
fact,  which  is  a  great  question  in  chemistry:  all  work 
must  be  so  arranged  that  it  can  be  carried  on  continu- 
ously. In  order  to  do  this,  what  is  required  must  be 
dehvered  regularly,  and  what  is  superfluous  must  be 
regularly  got  rid  of.     What  is  being  used  up  here  ? 

P.  The  water,  which  changes  into  steam. 

M.  Right.  Besides  that,  the  heat,  which  is  necessary 
to  make  steam.    And  what  is  superfluous? 

P.  The  warm  water  in  the  dish.  That  could  be  changed 
by  letting  it  out  through  a  siphon  and  filling  it  from 
above. 

M.  Good;  and  the  water  which  has  distilled  over  could 
be  replaced  in  the  flask  by  means  of  a  funnel. 

P.  But  the  steam  would  escape. 

M.  The  tube  need  only  be  dipped  under  the  water, 
and  then  it  is  closed.  But  our  cooling  could  be  improved, 
because  if  our  receiver  is  only  half  in  water,  the  upper  side 
remains  uncooled,  and  the  steam  won't  be  completely 
condensed. 

P.  It  must  always  be  turned  round  so  that  the  cool 
side  is  at  the  top. 

M.  Then  a  man  or  an  apparatus  is  needed  to  turn  it. 
We  must  have  a  cooler  that  does  all  that  is  necessary  itself. 

P.  Then  water  can  be  allowed  to  run  over  the  top 
side,  as  well  as  run  out  at  the  bottom. 

M.  That  is  better.  But  there  is  still  a  difficulty:  the 
running  cold  water  will  mix  with  the  warm  water  in 


IV/ITER.  155 

the  dish,  and  a  great  deal  of  cold  water  is  needed.  Can't 
that  be  bettered? 

P.  You  are  asking  a  great  deal ! 

M.  If  a  technical  or  scientific  exercise  has  to  be  done, 
you  must  never  be  contented  with  what  you  have  reached, 
but  must  always  ask:  Can  it  not  be  made  better?  And 
if  you  find  a  fault  or  incompleteness,  you  must  always 
ask:    How  can  I  improve  it? 

P.  I  can't  do  it. 

M.  It  is  possible  with  this  condenser  (Fig.  31).  It  is 
made  with  an  inner  tube  for  steam  and  an  outer  jacket 


Fig.  31. 

for  cold  water,  which  can  be  made  of  tin.  The  jacket 
is  provided  with  doubly  bored  corks  at  both  ends;  the 
steam-tube  passes  through  one  opening,  and  short  tubes 
are  stuck  through  the  others,  of  which  the  lower  one 
is  for  letting  in  and  the  upper  one  for  letting  out  the 
cold  water.  A  screw  clip  is  used  to  regulate  the  amount 
of  water  let  in;    the  hot  water  is  let  out  at  the  top. 

P.  Why  must  the  cold  water  come  in  from  under- 
neath? I  thought  it  would  cool  better  if  you  let  the  cold 
water  cool  the  steam  at  the  upper  end. 

M.  It  would  be  just  the  opposite,  because  it  would 
be  wasteful,  since  warm  water  is  lighter;  it  would  always 
rise  upwards  and  mix  with  the  cold  water.     But  when 


15<5  CONyERSATIONS  ON  CHEMISTRY. 

the  cold  water  comes  from  underneath  it  serves  to  con- 
dense the  remainder  of  the  steam.  It  gets  hotter  towards 
the  top,  and  gives  up  its  heat  as  thoroughly  as  possible 
for  coohng  purposes,  for  the  steam  which  enters  above 
is  condensed  by  the  nearly  boiling  water,  and  in  this 
way  the  coohng  water  is  most  thoroughly  used,  because 
1  useless  mixing  of  the  cold  water  and  warm  water  is 
ctvoided. 

P.  I  begin  to  see  how  many  things  you  have  to  think 
of  in  setting  up  a  small  apparatus. 

M.  This  is  the  first  instance  you  have  had  of  the 
principle  of  opposing  currents.  While  the  vapour  streams 
from  above  downwards,  and  loses  its  heat  more  and 
more,  the  cold  water  streams  from  below  upwards,  and 
absorbs  that  heat  regularly.  You  will  later  on  get  to 
know  a  great  number  of  other  cases  where  the  same 
principb  of  opposing  currents  is  employed.  Its  use  is 
accompanied  with  the  greatest  possible  economy. 

P.  I  can't  quite  understand  that,  but  I'll  try  to  remem- 
ber, so  that  I  may  look  out  for  other  instances  of  the 
same  kind. 

M.  Now  we  have  collected  some  distilled  water.  You 
can  convince  yourself  that  it  has  exactly  the  same  taste 
as  what  I  gave  you  before,  and  has  not  the  least  taste 
of  ink. 

P.  Why  does  it  taste  so  bad?  Well-water  has  no 
particular  taste,  yet  it  is  pleasant  to  drink. 

M.  Because  we  have  always  been  used  from  our  child- 
hood to  drink  well-water,  in  which  certain  foreign  sub- 
stances are  contained,  and  have  grown  accustomed  to 
it.  Pure  water  makes  a  different  impression  upon  our 
nerves  of  taste  from  well-water  and  we  call  it  unpleasant. 
Now  we  will  make  a  wash -bottle. 


IVATER. 


157 


Fig.  32. 


P.  What  is  a  wash-bottle  and  what  is  it  used  for? 

M.  We  must  use  pure  water  for  our  chemical  experi- 
ments in  order  not  to  mix  other  substances  with  our 
solutions.  We  keep  this  water 
in  a  vessel  in  order  that  we 
may  conveniently  use  it.  First 
I  cut  off  a  piece  of  glass  tub- 
ing half  as  long  again  as  the 
height  of  this  flask  ;^  and  then 
a  short  piece.  I  hold  the  long 
bit  in  the  flame  and  turn  it 
round  till  its  edge  softens;  it 
contracts,  and  when  the  open- 
ing is  reduced  to  half  a  milli- 
metre I  let  it  cool.  Then  I 
bend  the  short  tube  to  an 
obtuse  angle,  and  the  long  one,  after  the  end  has  cooled, 
to  an  acute  angle;  and  lastly  I  round  all  the  ends.  And 
now  I  bore  two  holes,  in  a  cork  that  fits  the  flask,  stick 
the  tubes  through  the  holes,  and  my  wash- bottle  is  ready 
(Fig.  32).  Now  we  will  fill  it  with  distilled  water,  after 
having  washed  it  out  several  times. 

P.  What  is  the  use  of  all  that? 

M.  When  I  blow  into  the  short  tube,  water  issues  out 
of  the  longer  one  in  a  thin  stream  which  I  can  direct 
where  I  like.  And  if  I  need  more  water,  I  turn  the 
flask  upside  down  and  a  pretty  large  stream  pours  out 
of  the  short  tube. 

P.  It  looks  to  me  as  if  you  had  taken  a  great  deal  of 
trouble  for  very  little  purpose. 

M.  Not  at  all;  for,  by  the  use  of  the  wash-bottle,  my 
daily  work  is  made  so  much  more  easy  and  certain  that 
my  trouble  is  soon   rewarded.      Every  mechanic   takes 


^58  CONVERSATIONS  ON  CHEMISTRY. 

care  to  provide  himself  with  the  best  possible  tools,  even 
though  they  are  dear;  he  is  repaid  with  ample  interest 
because  he  is  able  to  produce  more  and  better  work  in 
the  same  time.  For  the  chemist  a  wash-bottle  is  a  suit- 
able tool. 

P.  But  my  father  has  told  me  that  Benjamin  Franklin 
once  said  that  we  ought  to  be  able  to  bore  with  a  hammer 
and  saw  with  a  gimlet. 

M.  That  is  not  bad  advice;  it  means  that  one  should 
be  able  to  adapt  oneself  to  anything.  But  there  is  a 
great  difference  between  getting  over  a  difficulty  once, 
and  regular  work.  For  example,  I  njight  write  with  this 
match  dipped  in  ink,  if  I  had  no  pen,  but  as  I  can  write 
better  and  quicker  with  a  pen,  I  prefer  it.  But  we  have 
forgotten  our  water  all  this  time.  What  is  the  colour 
of  water? 

P.  I  don't  think  it  has  any.     It  is  colorless. 

M.  Yes,  in  thin  layers  it  appears  colorless,  but  in  thick 
layers  pure  water  is  blue. 

P.  What  is  the  reason  for  the  difference  ? 

M.  Water  is  so  faintly  colored  that  in  thin  layers  the 
color  is  not  recognizable.  But  you  learned  long  ago  that 
the  color  is  more  distinct  the  thicker  the  layer.  Pure 
water  in  a  white  bath  shows  the  blue  color  distinctly. 

P.  The  next  time  I  take  a  bath  I  will  look  out  for 
that.     But  the  water  in  the  river  is  not  blue,  but  brown. 

M.  The  reason  for  that  is  that  the  water  in  the  river 
contains  foreign  subtsances,  the  color  of  which  is  brown. 
Sea-water  is  generally  pure,  and  has  a  blue  color;  but 
if  it  is  mixed  with  brown  substances  the  mixture  looks 
green. 

P.  But  sea- water  is  not  in  the  least  pure,  for  it  cc  n- 
tains  salt. 


IVATER. 


159 


M.  Quite  right;  but  salt  is  colorless,  and  so  it  doesn't 
alter  the  color  of  the  water.  What  is  the  density  of 
water  ? 

P.  I  remember  that.  Its  density  is  i,  for  it  serves  as 
the  standard  of  density. 

M.  Good;  that  is,  its  density  at  4°C.:  at  all  other 
temperatures  it  is  less.  While  all  other  substances 
expand  by  heating,  water  contracts  between  0°  and  +  4°  C. 
And  above  that  temperature  it  expands. 

P.  I  should  hke  to  see  that.  ' 

M.  There  are  several  ways  of  showing  it.     Take  a 
wooden  bucket,  bore  a  hole  in  the  side  near  the  bottom, 
and    cork    a    thermometer 
into    the    hole.     Then    fill 
the    bucket  with  ice-water 
in  which  pieces  of  ice  are 
floating,   and   let    it    stand 
(Fig.  33).     After  some  time 
the  thermometer  below  will 
show  the  temperature  +  4° 
C,  while  another  thermom- 
eter dipped   in   the  water  at  the  top  will  stand  at  0°. 
Explain  that  to  me. 

P.  Because  water  at  4°  C.  is  heavier,  and  must  collect 
at  the  bottom. 

M.  Something  more  might  be  said  about  that,  but  it 
is  right  in  the  main. 

P.  How  would  this  do;  would  this  not  be  simpler? 
If  water  were  enclosed  in  a  thermometer-tube,  it  would 
contract  between  0°  and  4°,  and  then  rise  again.  Could 
not  a  water-thermometer  hke  that  be  made  ? 

M.  Yes,  of  course.     Here  I  have  a  glass  tube  of  pretty 


Fig.  2>Z' 


i6o 


CONVERSATIONS  ON  CHEMISTRY. 


F& 


narrow  bore,  about  half  a  millimetre.  I  heat  the 
end  till  it  meks,  and  blow  in;  I  make  a  bulb 
just  like  a  soap-bubble.  I  fasten  on  the 
upper  end  a  cork  with  a  wider  piece  of  tub- 
ing, which  I  fill  with  water  (Fig.  34).  I  first 
warm  the  bulb  slightly,  and  air-bubbles  escape 
through  the  water  above.  Then  I  cool  it 
down  again,  and  some  water  is  sucked  into 
the  bulb.  I  boil  this  water,  and  when  I  take 
away  the  flame  the  water  rushes  into  the 
bulb  and  fills  it.  Generally  a  small  air- 
bubble  remains,  but  that  is  easily  removed 
by  heating  the  water,  and  then  coohng  it; 
the  bubble  is  pushed  out,  and  on  cooHng  the 
water  fills  the  tube. 

P.  But  how  can  a  scale  be  fa^ened  on  ? 
M.  I  take  a  piece  of  an  old  millimetre  scale 
or  some  divided  paper,  or  something  of  that 
sort,  and  stick  it  on  the  tube  with  sealing-wax.  After 
my  water-thermometer  has  taken  the  temperature  of 
the  room,  I  remove  the  upper  tube.  Now  I  will  trust 
the  apparatus  in  your  hands,  and  also  a  thermometer. 
Tie  them  both  together  so  that  you  can  read  both  scales 
easily,  and  place  them  in  a  large  dish  of  water.  Now 
notice  where  the  mercury  in  the  thermometer  stands  and 
where  the  water  stands.  Now  put  some  ice  in  the  water 
so  that  the  temperature  sinks  about  a  couple  of  degrees, 
and  stir  it  for  at  least  five  minutes  till  the  water-ther- 
mometer has  got  steady,  and  write  down  the  readings 
you  find.  Go  on  till  the  temperature  is  nearly  0°.  Tell 
me  to-morrow  what  you  have  found. 


6 

Fig.  34. 


P.  I'm  afraid  that  what  I  have  done  is  not  worth  any- 


fVATER.  i6i 

thing.  I  played  the  whole  afternoon  with  the  thermom- 
eter, but  I  couldn't  find  out  that  the  volume  of  water  was 
smallest  at  4°. 

M.  What  did  you  find? 

P.  That  the  water  sinks,  to  begin  with,  as  the  ther- 
mometer falls,  but  at  about  8°  it  stops,  and  if  I  cool  it,  it 
rises  again.  I  always  get  8°  as  the  temperature  of  the 
smallest  volume. 

M.  What  do  you  think  is  the  reason  for  that  ? 

P.  I  didn't  think  of  looking  for  a  reason.  I  only 
thought  that  I  had  read  it  wrongly,  but  I  always  got  the 
same. 

M.  Then  your  readings  were  right.  What  quantity 
were  you  measuring? 

P.  The  volume  of  the  water. 

M.  No,  you  were  only  measuring  the  position  of  the 
water,  and  drawing  a  conclusion  from  that  as  to  its 
volume.  Before  you  can  draw  a  conclusion  from  the 
position  of  the  water  as  regards  its  volume,  you  must 
make  sure  that  the  capacity  of  the  thermometer-bulb 
always  remains  the  same.    Are  you  quite  sure  of  that? 

P.  Let  me  see.  Yes,  I  always  found  the  same  posi- 
tion at  the  same  temperature. 

M.  Very  good.  But  from  that  you  can  only  conclude 
that  at  the  same  temperature  the  capacity  was  the  same. 
Do  you  see  now? 

P.  You  mean  that  the  glass  of  the  bulb  had  expanded 
by  heat?  That  couldn't  make  any  difference,  because 
the  glass  is  so  thin  that  it  makes  only  a  very  small 
fraction  of  the  volume  of  the  water.  And  the  small 
expansion  of  this  small  volume  couldn't  make  a  great 
difference. 

M.  You  have   made   a  mistake   in   reasoning.     You 


1 62  CONyERSATlOm  ON  CHEMISTRY. 

have  supposed  that  the  alteration  of  the  volume  of  the 
glass  had  to  be  considered?  That  is  not  true.  You 
should  have  considered  the  increasing  volume  of  the 
glass  bulb,  which  is  the  same  as  that  of  a  solid  ball  of 
glass  of  the  same  size  as  our  thermometer-bulb,  and  is 
nearly  as  great  as  the  expansion  of  water. 

P.  But  the  ball  is  not  solid. 

M.  Think  of  a  solid  ball  heated  uniformly  to  any 
high  temperature;  will  the  interior  be  in  a  state  of  strain 
or  in  equilibrium? 

P.  I  think  it  will  be  in  equihbrium,  for  it  expands 
uniformly. 

M.  Right.  Now  think  of  this  ball  as  consisting  of 
a  number  of  hollow  balls  fitting  each  other  accurately, 
like  the  layers  of  an  onion ;  would  there  be  any  difference 
if  such  a  ball  were  heated? 

P.  I  see  no  reason.  Ah,  now  I  understand:  the  out- 
side layer  would  expand  exactly  as  if  the  inner  layers 
were  not  there,  just  as  if  it  were  a  sohd  ball.  That's 
very  ingenious. 

M.  Now  you  see  the  reason  why  you  found  the  point 
of  the  smallest  volume  of  the  water  too  high.  If  the 
water  hadn't  expanded  at  all  it  would  have  sunk  in  the 
stem,  because  the  volume  of  the  bulb  would  have  increased. 
It  is  only  when  the  expansion  of  the  water  is  exactly 
even  with  that  of  the  glass  that  it  remains  stationary 
in  the  stem;  and  that  is  at  8°.  As  you  see,  you  have 
been  examining  the  difference  between  the  expansions 
of  water  and  of  glass,  and  in  order  to  find  out  the  former 
you  must  know  the  latter,  but  that  is  not  easy  to  find  out. 

P.  Oh,  bother!  I  thought  I  was  doing  the  thing  well 
and  I  have  been  wasting  my  time. 

M.  Not  wasting,  because  you  have  learnt  how  much 


ICE,  163 

there  is  to  think  about  for  every  experiment  before  you 
know  how  to  interpret  it. 


20.  ICE. 

M.  Yesterday  you  learned  some  of  the  properties  of 
water;   which  do  you  remember  best? 

P.  The  greatest  density  of  water  and  the  experiment 
about  that.  I  tried  it  with  a  pail  and  it  came  out  all 
right. 

M.  Good.  It  is  important  in  nature  that  the  density 
of  water  has  its  greatest  value  at  4°;  that  is  called  the 
temperature  of  maximum  density. 

P.  Why  should  such  a  small  difference  be  so  important  ? 

M.  When  still  water,  for  instance  a  lake,  is  cooled  on 
the  surface,  the  colder  water  sinks  till  the  whole  of  the 
water  has  reached  the  temperature  of  4°.  Then  the 
cold  water  stays  above  till  it  freezes,  while  below  the 
temperature  4°  persists,  just  as  in  your  experiment  with 
the  pail  (page  159). 

P.  Then  fish  aren't  so  cold,  after  all? 

M.  That  is  of  no  great  importance;  but  if  it  were  not 
the  case,  ice  would  be  deposited  at  the  bottom  of  the  lake 
and  it  would  freeze  through  and  through,  instead  of 
being  coated  with  ice  only  on  the  surface.  The  fish  would, 
of  course,  die,  and  in  spring  it  would  be  much  longer 
before  all  the  ice  melted.  In  quickly  running  rivers  where 
the  water  is  thoroughly  mixed  it  sometimes  happens  in 
a  hard  winter  that  all  the  water  is  cooled  below  zero,  and 
then  ground-ice  is  formed,  which  floats  to  the  top  when 
its  mass  has  sufficiently  increased.    / 


164  CONyERSATIONS  ON  CHEMISTRY. 

P.  I  should  have  thought  that  the  lake  would  become 
covered  with  ice,  for  ice  floats  upon  water. 

M.  There  is  another  condition  which  protects  lakes  from 
freezing.  This  brings  us  to  the  properties  of  ice.  You 
know  that  water  changes  to  ice  at  0°.  But  now  I  will 
show  you  that  that  isn't  always  the  case.  I  mix  some 
pounded  ice  with  a  little  salt,  and  the  temperature  falls 
below  0°,  and  is  lower  the  more  salt  I  mix  with  the  ice. 
Now  give  me  your  water-thermometer  and  the  mercury- 
thermometer.  The  temperature  of  my  cold  mixture 
is  —  5°.  I'll  put  the  bulb  in  and  let  the  water  cool  itself 
down. 

P.  It  will  freeze  and  the  bulb  will  be  sure  to  burst. 

M.  Then  you  can  blow  a  new  one.  But  you  needn't 
be  afraid;  it  won't  freeze. 

P.  Why  is  that? 

M.  As  long  as  there  is  no  ice  present  water  can  be 
cooled  considerably  below  0°  without  freezing.  Only, 
when  you  bring  it  in  contact  with  some  ice,  the  water 
becomes  solid. 

P.  Why  .is  that? — I  beg  your  pardon,  I  must  ask 
the  question  differently :   What  else  is  it  connected  with  ? 

M.  That  is  a  difficult  question  to  answer.  Now 
remember  that  the  temperature  0°  always  persisted  when 
water  and  ice  were  simultaneously  present.  If  you 
cool  water  alone  below  0°  ice  may  be  formed,  but  it  is 
not  necessary  that  it  should  be.  That  is  a  general  state- 
ment; even  though  the  conditions  are  present  for  the 
formation  of  new  substances  or  forms,  these  do  not  gen- 
erally appear  of  their  own  accord,  but  the  point  of  change 
may  be  more  or  less  exceeded.  Only,  this  becomes  im- 
possible when  these  new  substances  are  present,  for  they, 
being  already  in  existence,  increase. 


ICE.  165 

P.  That  is  no  explanation;  it  is  only  a  description. 

M.  Quite  right.  You  know  now  under  what  cir- 
cumstances such  phenomena  become  manifest,  and 
what  their  relations  are.  What  more  do  you  wish? 
When  you  have  learnt  more  about  chemistry  you  will 
get  to  know  of  other  relations,  and  be  able  to  look  at 
these  phenomena  from  many  points  of  view.  That  is 
all  that  we  can  hope  to  learn  from  science,  and  it  is 
surely  enough.  In  order  that  we  may  be  able  to  talk 
of  such  things  in  future,  I  may  tell  you  that  what  you 
have  seen  with  water  is  supercooling;  a  more  general 
word  is  supersaturation. 

P.  I  see  I  have  a  great  deal  to  learn  still. 

M.  So  have  we  all.  Then  ice  floats  upon  water;  what 
conclusion  can  you  draw  from  that? 

P.  That  ice  is  lighter  than  water. 

M.  Do  you  mean  that  water  loses  weight  when  it  freezes  ? 

P.  No.  Water  which  is  displaced  by  ice  weighs  more 
than  the  ice. 

M.  When  the  ice  is  completely  immersed  in  the  water. 
Or  in  other  words,  when  water  freezes  the  resulting 
ice  occupies  a  greater  volume  than  that  previously  occu- 
pied by  the  water.  There  is  a  good  deal  of  difference: 
ten  volumes  of  water  give  more  than  eleven  volumes  of 
ice.  That  is  a  pecuHarity  of  water.  Most  other  sub- 
stances contract  on  freezing,  so  that  the  solid  sinks  in  the 
liquid. 

P.  Has  that  any  connection  with  the  expansion  of 
water  below  4°? 

M.  That  is  a  question  which  has  given  rise  to  much 
speculation.  But  no  satisfactory  explanation  has  yet 
been  found.  Have  you  ever  seen  water  beginning  to 
freeze  ? 


i66 


CONyERSATlONS  ON  CHEMISTRY, 


P.  Oh,  you  mean  when  only  a  httle  of  it  is  frozen? 
Yes,  long  needles  appear  on  the  surface.  I  have  often 
seen  it  on  the  puddles. 

M.  These  are  crystals,  for  ice  is  a  crystalline  body. 

P.  I  know,  I  have  often  seen  large  snow- crystals. 
They  look  like  stars  with  six  rays,  or  six-sided  plates. 

M.  Quite  right.  Here  are  some  photographs  of  snow- 
crystals  (Fig.  35).  The  frost  on  the  window-pane  con- 
sists of  ice- crystals. 


Fig.  35. 


P.  But  their  shape  isn't  regular. 

M.  Because  the  water  freezes  too  quickly  on  the  pane 
for  the  crystals  to  have  time  to  form.  But  sometimes 
where  the  pane  is  almost  quite  clear  you  will  see  pretty 
regular  crystals  which  have  deposited  slowly  from  the 
water  in  the  air. 

P.  Then  hoarfrost  also  consists  of  crystals? 

M.  Yes.  You  have  seen  them  glistening  in  the  sun, 
which  is  reflected  on  their  surfaces.  A  sheet  of  ice  on  a 
frozen  pond  can  also  be  shown  to  be  crystalHne.  Ice  is 
as  blue  as  liquid  water. 

P.  But  snow  is  quite  white!  Stop,  I  know  why; 
because  it  is  so  finely  divided  (page  9).     And  I  remem- 


ICE.  167 

ber  that  large  blocks  of  ice  that  you  see  on  the  street 
look  quite  blue. 

M.  Large  masses  of  ice  called  glaciers  slip  down  from 
high  hills  which  are  covered  with  everlasting  snow; 
when  they  move  they  split,  and  in  the  cracks,  or  crev- 
asses as  they  are  called,  they  look  beautifully  blue. 

P.  I  suppose  because  the  light  has  to  penetrate  through 
thick  layers  of  ice. 

M.  Quite  right.  Now  we  will  look  at  some  ice  melt- 
ing. I  take  a  thick  iron  plate  and  lay  it  upon  a  tripod 
above  the  spirit-lamp.  Now  I  take  two  similar  beakers 
or  flasks  and  put  some  ice  into  the  one,  and  into  the 
other  an  equal  weight  of  water  at  0°.  I  place  these 
beakers  symmetrically  on  the  plate,  so  that  each  gets 
the  same  amount  of  heat  from  below;  and  in  each  I 
place  a  thermometer.  Now  we  can  proceed  with  the 
experiment. 


Fig.  36. 

P.  What  is  to  be  learnt  from  that? 
M.  That  ice  can  absorb  a  lot  of  heat  without  becom- 
ing warmer. 


1 68  CONVERSATIONS  ON  CHEMISTRY, 

P.  How  can  that  be? 

M.  Look  here:  the  thermometer  in  the  water  has 
risen  from  o°  to  20°.     The  one  in  the  ice  is  still  at  0°. 

P.  The  reason  for  that  must  be  that  ice^  is  present  with 
the  water,  and  therefore  the  temperature  must  stay 
at  0°. 

M.  Quite  right;  as  much  heat  must  have  entered  the 
ice  as  was  necessary  to  raise  the  water  from  0°  to  20°, 
and  yet  the  ice  is  no  warmer.  What  has  happened  to 
the  ice  ? 

P.  Some  of  it  has  melted.  So  heat  is  really  used  up 
by  the  melting  of  ice? 

M.  Exactly.     What  is  heat? 

P.  One  kind  of  energy  or  work.  So  it  requires  work 
to  change  ice  into  water. 

M.  Quite  right.  Before  people  had  acquired  a  con- 
ception of  energy,  they  were  very  much  surprised  at  this, 
and  said  that  although  heat  was  not  perceptible  to  the 
thermometer  it  must  nevertheless  be  present,  and  only 
lay  concealed;  they,  therefore,  called  this  heat  latent 
heat,  from  lateo,  I  lie  hid.  Even  now  this  name  is 
used,  although  the  former  false  conceptions  have  been 
replaced  by  correct  ones. 

P.  I  don't  quite  understand  that. 

M.  You  know  that  in  general,  work  or  energy  is  used 
up  in  producing  a  change  of  state;  so  it  is  here.  For 
example,  when  you  grind  a  piece  of  sugar  to  powder  you 
can't  do  that  without  work,  just  as  when  you  break 
a  rod  or  bend  a  wire.  In  the  same  way  melting  requires 
work,  and  this  work  is  derived  from  simple  addition 
of  heat. 

P.  Can  the  work  be  done  in  any  other  way? 

M.  Certainly.    If  two  pieces  of  ice  at  c°  are  rubbed 


ICE.  169 

together  they  melt.  Now  the  ice  has  melted  and  the 
thermometer  has  risen  to  a  little  above  0°.  The  other 
thermometer  shows  nearly  80°.  Now  notice.  The 
amount  of  heat  required  to  raise  i  gram  of  water  through 
1°  is  called  a  calory,  abbreviated  cal.  To  raise  i  gram 
of  water  from  0°  to  80°,  80  cals.  are  required;  to  raise 
200  grams  of  water  to  30°,  200X30  =  6000  cals.  are  re- 
quired. The  amount  of  heat  is  measured  by  multiply- 
ing the  rise  of  temperature  by  the  weight  of  the  water. 

P.  I  understand  that.     But  if  the  water  grows  colder? 

M.  Then  heat,  equal  to  the  product  of  the  lowering  of 
temperature  multiplied  by  the  weight  of  the  water,  has  es- 
caped. Now  the  water  has  become  80°  warmer  by  absorb- 
ing the  heat  required  to  melt  an  equal  weight  of  ice;  so 
that  each  gram  of  water  has  absorbed  80  cals.  and  each 
gram  of  ice  exactly  the  same  quantity.  And  it  follows 
that  each  gram  of  ice  requires  80  cals.  in  order  to  melt 
to  water  at  0°.  In  other  words,  80  cals.  are  the  work 
of  melting,  or  the  heat  oj  jusion  of  ice.  The  old  name 
which  I  explained  to  you  before  is  the  latent  heat  of  water. 

P.  But  this  number  refers  to  i  gram  of  ice. 

M.  Quite  right.  Such  numbers  are  generally  referred 
to  unit  of  weight,  because  then  it  is  only  necessary  to 
multiply  by  the  weight  in  order  to  find  the  value  for  a 
given  quantity.  Let  us  make  an  application  of  this. 
We  will  weigh  500  grams  of  water  into  a  beaker,  and 
after  measuring  the  temperature  with  a  fine  thermom- 
eter we  will  drop  in  a  piece  of  ice.  The  temperature 
is  18.7°  and  the  ice  weighs  34  grams.  Now  I  put  the 
ice  into  the  water,  and  stir  it  carefully  with  the  ther- 
mometer until  all  the  ice  has  melted.  The  thermometer 
has  fallen  to  12.4°.  You  can  calculate  the  latent  heat 
of  water  from  this. 


17°  CONVERSATIONS  IN  CHEMISTRY, 

P.  I'll  try.  500  grams  of  water  have  lost  18.7  —  12.4= 
6.3O  so  that  500X6.3  =  3150  cals.  have  been  used.  That 
heat  melted  34  grams  of  ice,  so  that  each  gram  took 
93  cals.     Is  that  right? 

M.  Pretty  nearly,  but  not  quite.  By  heat  of  fusion  is 
meant  the  heat  required  to  change  i  gram  of  ice  at  0° 
into  vi^ater  at  0°.  The  ice-water,  however,  gets  heated 
up  with  the  rest  of  the  water  to  12.4°,  so  that  you  have 
calculated  the  heat  of  fusion  higher  than  it  should  be. 

P.  Yes,  I  see  that,  but  how  can  it  be  corrected? 

M.  By  bringing  everything  that  happens  into  the 
calculation.  You  were  right  in  supposing  that  500  grams 
of  water  had  lost  500X6.3  =  3150  cals.  But  of  this 
34X12.4  =  422  cals.  have  been  used  in  warming  the 
melted  ice,  and  only  the  difference,  3150—422  =  2728, 
has  been  used  for  melting  the  ice.  This  difference 
divided  by  34  gives  80  cals.  as  the  heat  of  fusion  of  ice. 

P.  I  see  again  that  making  experiments  is  much 
easier  than  drawing  the  right  conclusions  from  them. 

M.  But  even  here  we  haven't  taken  everything  into 
consideration.  We  have  paid  no  attention  to  the  fact 
that  not  only  the  500  grams  of  water  were  cooled  down, 
but  also  the  thermometer  in  the  beaker.  Then  we 
just  noticed  that  the  beaker  with  the  cold  water  is  grad- 
ually warming  itself  in  this  room,  so  that  while  the  ice 
was  being  melted,  heat  was  entering  from  outside,  and 
the  lowering  of  temperature  was  therefore  too  small. 
Even  that  is  not  all  that  we  ought  to  have  considered, 
but  I  will  refrain  from  adding  more  so  as  not  to  confuse 
you. 

P.  I'm  rather  muddled  as  it  is,  and  can't  understand 
how  there  are  people  who  know  all  these  things  and  can 
do  them  correctly. 


STEAM.  171 

M.  You  can  neither  use  a  lathe  nor  paint,  and  while 
you  were  learning  to  cycle  you  found  that  very  difficult. 
To  make  correct  measurements  is  an  art  in  itself  which 
has  to  be  learnt,  and  no  one  ever  learns  it  thoroughly. 
Exact  measurements  prove  the  heat  of  fusion  of  ice  to 
be  81  cals. 

21.  STEAM. 

M.  To-day  we  shall  speak  about  steam. 

P.  Water  again!  If  we  are  going  to  spend  as  much 
time  over  other  substances,  I  shall  never  have  done 
with  chemistry. 

M.  Water  is  only  an  example  by  means  of  which 
we  learn  the  behaviour  of  substances  under  different 
circumstances.  All  the  so-called  laws,  for  example  those 
relating  to  melting  and  solidifying,  are  the  same  with 
other  substances,  so  that  you  don't  need  to  learn  them 
again.  -=,_ 

P.  But  why  did  we  choose  water  as  an  example? 

M.  Because  water  has  been  more  studied  than  any 
other  substance  and  is  therefore  best  known. 

P.  But  why  was  water  so  carefully  studied? 

M.  Because  it  occurs  in  such  large  quantity  on  the 
earth.  Just  think  how  differently  the  surface  of  the 
earth  looks  when  the  temperature  is  below  0°.  Of 
course  the  reason  is  that  water  freezes  at  0°.  The  dif- 
ference isn't  merely  the  appearance  of  ice  and  snow,  but 
also  the  temporary  cessation  of  life  in  plants,  caused  by 
the  fact  that  the  sap  can  no  longer  flow. 

P.  Yes,  I  see  that  w^ater  affects  almost  everything. 

M.  Besides,  since  water  is  so  abundant,  it  is  easier  to 
obtain  purer  than  other  substances,  and  so  it  is  especially 


172  CONyERSATIONS  ON  CHEMISTRY. 

adapted  as  a  standard  with  which  to  compare  proper- 
ties. You  have  learned  this  use  of  water  already  for 
the  thermometer  and  for  density;  and  for  many  other 
properties  water  serves  as  a  standard.  .And  that  of 
itself  is  a  reason  for  getting  to  know  the  properties  of 
water  more  thoroughly  than  that  of  other  substances. 
We  shall  therefore  take  up,  to  begin  with,  the  boiling  of 
water. 

P.  Is  there  anything  special  about  that  ?  I  have  been 
taught  that  water  boils  at  ioo°  C.  whether  a  large  or  a 
small  flame  is  below  it. 

M.  Let  us  see.  I  boil  some  water  in  a  flask  and  close 
the  mouth  with  a  cork  while  it  is  boiling;  what  will 
happen  ? 

P.  The  pressure  of  the  vapour  will  rise  and  the  flask 
will  burst. 

M.  Quite  right,  so  I  take  the  flame  away  and  let  it 
cool  down.  But  as  it  cools  too  slowly,  I  will  pour  some 
water  on  the  flask.     What  do  you  see  ? 

P.  How  funny!     The  water  is  beginning  to  boil  again. 

M.  I  pour  more  water  on  it  and  the  boihng  begins 
again.  Now  it  is  so  cold  that  I  can  hold  it  in  my  hand 
without  burning  it;  the  water  can't  be  hotter  than  50° 
and  still  it  boils  whenever  I  pour  cold  water  on  the  top 
of  the  flask. 

P.  I  can't  believe  that  in  the  least. 

M.  Why  not?     You  must  beheve  what  you  see. 

P.  But  I  was  taught  that  water  boiled  at  100°,  and 
now  it's  boihng  at  a  much  lower  temperature. 

M.  Well,  what  conclusion  do  you  draw? 

P.  That  water  can  boil  at  all  possible  temperatures. 
But  that  is  nonsense. 

M.  Why? 


STEAM.  173 

P.  Because  the  water  always  showed  the  tempera- 
ture 100°  whether  the  flame  was  big  or  small. 

M.  Quite  right.  But  when  you  see  that  a  phenom- 
enon changes  you  must  conclude  that  some  condition, 
closely  connected  with  the  phenomenon,  has  changed. 
Now  think  what  is  the  difference  between  the  boiHng, 
now  and  before? 

P.  Heating  made  the  water  boil,  but  now  it  boils 
when  it  is  cooled. 

M.  It  can't  be  only  cooling,  because  then  water  would 
always  keep  on  boiling  when  you  took  away  the  flame. 
Is  there  nothing  else  you  can  think  of? 

P.  Yes,  you  corked  the  flask.  But  how  can  a  cork 
make  the  water  boil? 

M.  Take  the  cork  out. 

P.  That's  not  easy.  And  it  hisses  as  if  air  were  being 
sucked  in. 

M.  So  there  is  a  vacuum  in  the  flask.  Now  think 
why. 

P.  I  begin  to  see.  The  steam  blew  out  the  air,  and 
then  you  corked  the  flask  so  that  no  more  air  could 
enter. 

M.  Quite  right.  The  flask  contained  only  water  and 
steam,  and  when  I  poured  cold  water  on  it,  the  steam 
was  condensed,  the  pressure  was  lowered,  and  the  water 
was  obliged  to  boil. 

P.  But  does  water  really  form  steam  at  every  tempera- 
ture if  the  pressure  is  lowered? 

M.  Water  boils  at  every  pressure,  and  a  definite 
temperature  corresponds  to  each  pressure.  It  boils  at 
100°  only  when  the  pressure  is  exactly  that  of  the  at- 
mosphere. On  high  mountains  where  the  pressure'  is 
much  lower  boih'ng  water  is  not  hot  enough  to  cook  meat 


174 


CONVERSATIONS  ON  CHEMISTRY 


P.  I  should  like  to  see  that. 

M.  I   can  show  you  something  like  it.    I   close  the 
flask  with    a    perforated   cork    through    which    passes 

a  doubly-bent  glass  tube, 
one  of  the  limbs  of  which 
is  80  cm.  long  (Fig.  37). 
Its  end  dips  in  a  basin 
of  mercury.  Now  I  heat 
the  flask  again;  and  you 
can  hear  the  air  bub- 
bling through  the  mer- 
cury. Now  the  noise 
changes;  it  rings  almost 
like  a  metal. 

P.  What  is  the  reason  of 
that? 

M.  The    steam    is    now 

almost  quite  free  from  air, 

and  when  it  passes  into  the 

cold    mercury    it    suddenly 

changes    to    liquid     water, 

and  the  sides  of  the  bubble 

hit  each  other.     As  long  as 

air  was   present    the   sides 

couldn't  come  together,  but 

now    it    is    metal    hitting 

Fig.  37.  metal.     Now    I    take    the 

flame  away  and  you  see  that  when  I  pour  on  cold  water, 

the  boihng  begins  again. 

P.  What  is  the  use  of  the  tube  that  dips  in  the  mercury  ? 
M.  Notice  what  happens  when  I  pour  on  cold  water, 
the  flask. 

P.  At  first  the  mercury  goes  up  quickly,  and  then  it 


^ 


J 


STEAM.  175 

falls  when  the  boiling  begins;  but  I  see  it  stands  higher 
than  at  the  beginning. 

M.  Now  you  see  everything  happens  as  I  told  you  it 
would.  The  higher  the  mercury  is  sucked,  the  smaller 
the  pressure  on  the  inside  of  the  flask.  It  was  highest 
immediately  after  I  had  poured  water  on  it;  but  when 
the  water  began  to  boil  steam  was  formed,  which  again 
filled  the  space  and  increased  the  pressure,  and  the  mer- 
cury fell. 

P.  But  why  did  the  level  of  the  mercury  always  become 
higher  each  time? 

M.  Because  the  water  in  the  flask  became  colder 
each  time  I  poured  water  over  it,  and  its  vapour  pressure 
decreased.  It  boiled  only  when  the  pressure  was  made 
still  smaller. 

P.  So  that  boiling  takes  place  when  the  pressure  on 
the  water  is  less  than  the  vapour  pressure.  I  see  you 
nodding,  so  that's  rignt.  But  what  is  vapour  pressure? 
There  is  never  anything  but  vapour  in  the  flask. 

M.  Imagine  an  empty  space;  of  course  there  is  no 
pressure  in  it.  Now  introduce  some  water;  part  of  the 
water  changes  to  steam.  This  goes  on  till  the  space  is 
filled  with  steam  to  a  certain  extent,  and  then  evaporation 
stops.  And  vapour  is  formed  till  it  has  a  certain  definite 
density  in  the  space,  and  exerts  a  definite  pressure. 
The  density  and  the  pressure  depend  only  on  the 
temperature.  At  0°  the  pressure  is  very  small  and  could 
only  raise  the  mercury  4  mm.  high;  but  at  100°  it  is 
so  great  that  it  can  overcome  the  whole  pressure  of  the 
atmosphere. 

P.  And  above  100° ?     Can  water  be  made  hotter? 

M.  Certainly;  only  then  the  pressure  must  be  increased 
by  confining  the  vapour.     As  you  know,  this  occurs  in 


1 7  6  CON  VERS  A  TIONS  ON  CHE  MIS  TR  Y. 

a  boiler.  If  the  pressure  is  twice  as  high  as  the  atmos- 
pheric pressure,  the  temperature  of  the  water  is  121°. 
And  if  its  temperature  is  180°  the  pressure  is  ten  times 
as  great.  The  steam-engine  is  contrived  to  utihze  this 
pressure.  You  can  see  on  every  steam-boiler  an  ap- 
paratus which  looks  something  like  a  clock-face  and  is 
called  a  gauge,  or  a  manometer;  it  measures  the  pres- 
sure. 

P.  I  have  often  seen  it.  Why  is  the  pressure  meas- 
ured in  pounds? 

M.  It  means  pounds  per  square  inch.  The  pressure 
of  the  atmosphere,  that  is,  the  pressure  which  the  air 
exerts  upon  the  surface  of  the  earth,  is  16  lbs.  per  square 
inch.  But  steam  is  used  for  heating,  as  well  as  for  driv- 
ing, steam-engines.     Do  you  know  why? 

F.  Because  its  temperature  is  100°. 

M.  That  is  not  all ;  it  can  give  up  far  more  heat  than 
water  at  100°. 

P.  That  is  the  same  as  with  water  and  ice. 

M.  Quite  right.  To  change  water  at  100°  into  steam 
of  the  same  temperature  requires  a  great  deal  of  work 
which  can  be  added  in  the  form  of  heat.  We  can  make 
an  approximate  measurement.  We  will  first  heat  a 
known  weight  of  water  over  the  lamp,  and  by  measuring 
the  time  we  will  calculate  from  its  amount  and  from  the 
rise  of  temperature  how  much  heat  the  lamp  gives  it 
per  minute.  Then  we  will  boil  the  water  over  the  same 
lamp  and  measure  the  time  that  it  boils;  then  we  will 
weigh  it  again,  and  from  its  loss  we  will  find  how  much 
vapour  has  been  formed;  then  we  can  calculate  *  how 
many  calories  are  required  to  evaporate  i  gram. 

P.  I  should  like  to  do  that.  What  sort  of  dish  shall  I 
take? 


STEAM,  177 

M.  Take  a  flask;  we  shall  weigh  out  200  grams  of 
water.  Now  we  will  put  a  thermometer  into  it;  its 
temperature  is  i3°.  The  lamp  has  been  burning  for 
some  time  and  the  flame  has  become  regular;  I  place  it 
below  the  flask,  and  let  it  burn  for  fifteen  minutes.  Now 
what  is  the  temperature?  Remember  to  stir  before  you 
read. 

P.  78°.  That  is  60°  in  fifteen  minutes  or  4°  a  minute. 
As  there  were  200  grams  of  water,  the  lamp  gives  800  cals. 
a  minute. 

M.  Quite  right.  Now  the  water  begins  to  boil  and 
I  look  at  my  watch  again.  In  ten  minutes  I  take  the 
lamp  away  and  let  the  flask  cool.  On  weighing  it,  it 
has  lost  14  grams.  How  many  calories  does  that  require 
to  produce  i  gram  of  steam? 

P.  Ten  minutes  for  800  cals.  makes  8000  cals.,  and 
dividing  by  14  gives  571  and  a  fraction. 

M.  Fairly  good.  The  right  number  is  537  cals. 
The  reason  why  we  found  it  too  high  is  that  the  flask 
while  it  was  at  100°  has  been  losing  more  heat  than 
while  it  was  being  heated  up  from  18°  to  68°o 

P.  Yes,  I  can  quite  imagine  that  all  kinds  of  things 
must  be  thought  of  in  order  to  get  correct  numbers. 

M.  That  is  true;  the  measurements  are  much  more 
difficult  than  with  ice.  But  we  won't  trouble  about  that 
at  present.  As  you  see,  the  heat  of  evaporation  of  water  is 
nearly  seven  times  as  great  as  the  heat  of  fusion  of  ice. 

P.  Yes,  the  heat  of  fusion  was  81  cals. 

M.  For  this  reason  steam  can  be  used  to  convey  heat 
from  one  place  to  another  without  the  necessity  of 
carrying  much  weight.  The  vapour  is  produced  in  a 
boiler,  and  led  through  pipes  to  where  the  heat  is  wanted. 
In  schools  and  public  buildings,  heating  with   steam  is 


1 7^  CON  VERSA  TIONS  ON  CHE  MIS  TR  Y. 

often  adopted,  and  on  turning  a  stop-cock,  you  can 
make  it  either  cold  or  hot. 

P.  But  when  the  steam  has  given  up  its  heat,  it  goes 
back  to  Hquid  water.     What  becomes  of  the  water? 

M.  It  is  led  back  to  the  boiler.  The  water  makes 
a  circular  tour  through  other  pipes;  but  the  heat 
goes  from  the  boiler  to  those  places  where  it  is  wanted 
and  stays  there.  It  is  much  the  same  as  when  the 
piston  of  a  locomotive  conveys  motion  from  the  engine 
to  the  wheel,  and  again  comes  back;  but  the  work  stays 
there. 

P.  Is  there  steam-heating  in  railway  carriages  ?  One 
often  sees  steam  escape  between  the  carriages  in  winter. 

M.  Yes,  the  steam  which  has  escaped  from  the  cylinder 
of  the  locomotive  is  used  for  that  purpose,  after  it  has 
done  its  work. — So  now  we  know  water  in  all  its  three 
forms,  but  we  have  not  nearly  done  with  it.  Of  its 
other  properties,  perhaps  the  most  important  for  us  is  its 
power  of  dissolving  substances.  Do  you  remember  what 
you  learned  about  that? 

P.  That  water  becomes  saturated  when  it  dissolves 
anything. 

M.  Be  more  exact. 

P.  When  you  put  into  water  anything  it  can  dissolve, 
only  a  definite  quantity  goes  into  solution.  And  when 
the  water  can  dissolve  no  more,  it  is  said  to  be  saturated. 

M.  But  suppose  you  were  to  take  three  times  as  much 
water? 

P.  Then  three  times  as  much  substance  would  dissolve. 

M.  Quite  right,  but  that  is  only  at  some  definite  tem- 
perature.    If  you  were  to  heat  the  solution — 

P.  Then  it  would  dissolve  more. 

M.  That  is  not  always  right.     It  is  true  that  most 


STEAM.  179 

substances  behave  in  that  way,  but  there  are  some  that 
dissolve  equally  well  at  different  temperatures.  Common 
salt  is  such  a  substance;  it  is  nearly  equally  soluble  in 
hot  and  cold  water. 

P.  Are  there  any  substances  which  dissolve  better  in 
cold  than  in  hot  water? 

M.  There  are,  but  they  are  rare. 

P.  Which  substances  dissolve  in  water,  and  which  don't  ? 

M.  Strictly  speaking  all  substances  dissolve  in  water; 
but  there  are  many  which  dissolve  so  slightly  that  very 
accurate  tests  are  necessary  to  find  out  that  they  do. 

P.  Surely  glass  is  not  soluble  in  water? 

M.  Yes,  indeed;  although  it  is  only  very  sparingly 
soluble. 

P.  How  can  I  see  that? 

M.  Take  some  beet-root  juice  and  put  it  on  a  piece 
of  glass.  It  stays  red.  But  if  you  powder  the  glass  in  a 
mortar  along  with  beet-root  juice,  the  juice  turns  blue 
and  green.  The  reason  is  that  the  glass  dissolves  and 
acts  upon  the  juice  so  as  to  turn  it  green. 

P.  Why  must  the  glass  be  ground  up  in  a  mortar? 

M,  The  solution  takes  place  faster  when  the  surface 
is  increased. 

P.  I  hadn't  thought  of  that.  But  stones  don't  dissolve 
in  water. 

M.  All  river-  and  well-waters  contain  dissolved  sub- 
stances. You  can  see  that  from  the  deposit  in  the  tea- 
kettle where  the  dissolved  substances  settle  out  as  a 
grey  crust  which  is  called  fur. 

P.  Yes,  I  have  just  seen  them  removing  that  fur.  It 
stuck  very  tight. 

M.  Well,  these  dissolved  substances  came  from  the 
rocks  through  which  the  water  flowed  before  it  reached 


l8o  CONyERSATIOm  ON  CHEMISTRY. 

the  surface.  For,  lo  begin  with,  the  water  was  pure 
distilled  water. 

P.  How  could  that  be?    Who  distilled  it? 

M,  Well-water  comes  from  rain  which  falls  on  the 
surface  of  the  earth,  sinks  in,  and  collects  in  deeper 
places.     Where  does  rain  come  from  ? 

P.  From  the  clouds. 

M.  Yes,  and  clouds  are  formed  by  the  condensation  of 
water- vapour  out  of  the  air.  So  that  rain-water  is  really 
distilled  water,  indeed  quite  freshly  distilled.  When  you 
see  it  it  has  generally  run  over  a  roof,  and  carries  all 
the  dirt  with  it  that  has  collected  on  that  roof  since  the 
last  shower.     How  does  water  come  into  the  clouds? 

P.  It  evaporates  from  the  surface  of  the  earth,  and 
is  driven  about  by  the  wind. 

M.  That  is  right  so  far,  but  to  evaporate  it  recjuires 
heat,  and  you  have  just  measured  how  much.  Where 
does  it  get  the  heat  from? 

P.  Is  it  from  the  sun? 

M.  It  is.  Since  the  rays  of  the  sun  warm  whatever 
they  fall  upon,  they  also  form  a  kind  of  energy  which 
is  called  light,  or  radiant  energy.  The  sun  gives  the 
work  which  evaporates  the  water,  and  lifts  the  vapour 
into  the  air.  When  the  water  falls  again  as  rain  or 
snow,  the  work  is  partially  restored;  for  example,  it  can 
drive  a  mill. 

P.  So  mills  are  really  driven  by  the  sun? 

M.  Yes,  for  if  it  stopped  shining  all  streams  would 
stop  running.  Besides,  windmills  are  driven  by  the 
sun,  for  wind  is  the  result  of  its  action. 

P.  How  it  all  hangs  together!  1  look  upon  the  sun 
and  the  rain  with  quite  different  eyes  now. 

M.  You    will    learn    many    more    such    connections. 


STEAM,  l8i 

Now  let  us  get  back  to  the  property  which  water  has  of 
dissolving  substances.  When  water  has  dissolved  any 
substance,  it  is  said  to  form  a  solution.  Such  solutions 
are  much  more  in  use  than  the  substances  themselves. 

P.  Why? 

M.  Because  of  their  chemical  action.  Solid  sub- 
stances do  not  act  at  all  on  each  other,  or  only  very 
slowly  and  incompletely.  In  order  that  they  may  act 
upon  each  other  chemically  they  must  be  brought  together 
in  the  Hcjuid  state.  This  can  happen  in  one  of  two  ways; 
they  may  be  melted  or  they  may  be  dissolved.  Melting 
requires  a  high  temperature,  as  a  rule,  which  is  not  easily 
attained,  whereas  it  is  easy  to  make  a  solution.  More- 
over, many  suljstances  do  not  stand  a  high  temperature 
without  changing. 

P,  I  begin  to  see  that  water  is  almost  the  most  impor- 
tant thing  in  the  whole  of  chemistry. 

M.  Not  alone  in  chemistry,  but  also  in  daily  lifei  All 
food  contains  more  or  less  water;  tea,  coffee,  milk,  wine, 
beer,  are  solutions  and  to  some  extent  mechanical  mix- 
tures of  various  substances  in  water;  blood  and  all  other 
juices  of  the  body  are  also  aqueous  solutions.  So  is  the 
sap  of  plants;  you  know  that  every  plant  dies  when  it  is 
dried,  that  is,  when  water  is  removed  from  it.  And 
that  is  also  the  case  with  animals. 

P.  I  shouldn't  have  dreamt  that  water  was  such  an 
important  substance;  you  might  even  say.  No  water,  no 
life! 

M.  Of  course  you  can  also  say:  No  oxygen,  no  life;  no 
nitrogen,  no  life;  no  iron,  no  life,  and  so  on.  Life  is 
such  a  very  complicated  affair  that  many  conditions 
must  be  fulfilled  before  it  occurs.  You  can  picture  it 
as  a  stretched  chain  consisting  of  different  kinds  of  links; 


1 82  CONVERSATIONS  ON  CHEMISTRY, 

when  one  of  the  links  is  broken,  the  chain  breaks,  no 
matter  how  strong  the  other  links  are.  In  the  same  way 
life  stops  if  any  one  of  the  necessary  factors  is  wanting, 
so  that  none  of  them  can  be  called  the  most  important. 

22.  NITROGEN. 

M.  To-day   we  shall  learn  something  more  about  air. 

P.  We  are  taking  up  all  the  elements  one  after 
another;   first  fire,  then  water  and  earth,  and  now  air. 

M.  The  old  Greeks  called  them  elements  because  they 
were  universal  and  their  importance  could  not  be  over- 
looked. And  as  we  too  are  considering  the  most  impor- 
tant things  we  naturally  come  to  them.  What  do  you 
know  about  air? 

P.  That  it  is  a  gas,  but  not  an  element ;  it  is  a  mixture 
of  one-fifth  of  oxygen  and  four-fifths  of  another  gas — 

M.  Which  is  called  nitrogen.  I  told  you,  too,  that 
nitrogen,  like  oxygen,  has  neither  color,  odor,  nor  taste, 
and  that  it  differs  from  oxygen  in  not  supporting  com- 
bustion. It  is  not  combustible,  and  so  differs  from  hy- 
drogen. 

P.  Then  does  nitrogen  combine  neither  with  oxygen 
nor  with  other  substances  ? 

M.  Not  as  a  rule;  nitrogen  is  an  unsociable  character — 
it  likes  solitude,  doesn't  care  to  unite  wdth  other  elemen's, 
and,  even  after  it  has  united,  it  separates  as  soon  as  it  can. 
That  is  the  reason  why  air  consists  mostly  of  uncombined 
nitrogen.     For  as  it  is  a  gas  there  is  no  other  place  for  it. 

P.  Doesn't  it  dissolve  in  water? 

M.  Very  little,  even  less  than  oxygen.  We  will  make 
some  nitrogen.     How  can  that  be  done? 

P.  We  require  only  to  separate  nitrogen  from  the  air. 


NITROGEN. 


183 


M.  Quite  right;    how  shall  we  do  that? 

P.  Oh,  I  suppose  by  burning  something  in  the  air; 
a  candle? 

M.  That  has  several  disadvantages.  First  of  all,  other 
gases  are  produced  at  the  same  time  which  remain  mixed 
with  the  nitrogen ;  and  second,  a  candle  goes  out  long  before 
the  whole  of  the  oxygen  has  been  removed.  Here  is 
another  means  of  removing  oxygen:  it  is  phosphorus 
(see  page  105).  It  has  the  property  of  using  up  all  oxygen 
even  at  ordinary  temperature.  I  place  in  a  test-tube  a 
piece  of  phosphorus  which  has  been  made  fast  to  a  wire  by 
melting.  Then  I  invert  the  test-tube  over  water  (Fig.  38). 
You  see  that  a  white  vapor  flows  down  from  the  phos- 


Fig.  38. 

phorus;  it  consists  of  the  products  of  oxidation  and  con- 
tains oxygen.  At  the  same  time  the  water  begins  to  ri^e 
slowly,  and  after  about  an  hour  the  cloud  is  no  longer 
visible,  showing  that  all  oxygen  has  been  used;  a  fifth  of 
the  air  has  disappeared.  I  have  here  a  flask  in  which 
phosphorus  has  been  kept  since  yesterday.  It 
tains  only  nitrogen. 
P.  It  looks  exactly  like  air. 


now  con- 


1 84  CONVERSATIONS  ON  CHEMISTRY. 

M.  You  will  soon  see  that  it  is  not.  I  dip  in  a  burning 
splinter,  and  it  goes  out  just  as  if  it  were  plunged  into  water. 

P.  Give  me  some  phosphorus;  I  want  to  repeat  that 
experiment. 

M.  I  would  rather  not,  for  phosphorus  catches  fire 
very  easily,  and,  moreover,  is  very  poisonous.  I  will 
tell  you  another  plan.  There  is  a  compound  of  iron,  a 
sulphate;  it  is  a  green  salt.  If  you  dissolve  it  in  water 
and  mix  the  solution  with  lime  you  get  a  thin  paste  which 
takes  up  oxygen  very  quickly.  I  will  make  such  a  paste 
in  this  large  flask,  and,  after  I  have  corked  it,  I  shake 
it  thoroughly.  If  I  place  its  neck  under  water  and  take 
out  the  cork,  water  enters — a  sign  that  some  of  the  air  has 
disappeared. 

P.  Let  me  try  with  this  splinter.  Quite  right.  It  has 
gone  out. 

M.  There  is  not  much  more  to  show  you  with  nitrogen, 
as  it  does  not  combine  with  any  common  substances. 

P.  Is  it  light,  Hke  hydrogen? 

M.  No;  since  it  is  the  chief  constituent  of  the  air  it 
has  about  the  same  density  as  air.  It  -is  lighter  than 
air  because  oxygen  is  a  little  heavier. 

P.  Then  nitrogen  appears  to  be  a  sort  of  indifferent 
element,  which  is  unnecessary  for  changes  on  the  earth. 

M.  No,  that  is  by  no  means  the  case.  Nitrogen  is 
equally  important  in  peace  and  in  war,  first  because  it 
is  a  constant  constituent  of  all  living  creatures,  animals 
as  well  as  plants;  and  second  because  compounds  of 
nitrogen  form  gunpowder,  artificial  dyes,  and  innumer- 
able other  substances  which  are  equally  important  for 
industry  and  daily  life.  While  free  nitrogen  costs  nothing, 
because  you  can  have  as  much  as  you  like  of  it  in  the 
air,  combined  nitrogen  has  a  fairly  high  value;  it  costs 
about  lo  cents  a  pound. 


NITROGEN,  185 

P.  Why  don't  they  make  compounds  using  the  nitro- 
gen of  the  air? 

M.  You  will  find  a  serious  difficulty  there.  Making 
the  free  nitrogen  of  the  air  combine  with  other  sub- 
stances is  such  an  expensive  operation  that  the  price  of 
the  compound  is  prohibitive. 

P.  How  can  that  be?  It  costs  nothing  to  change 
oxygen  or  hydrogen  into  compounds;  the  change  happens 
by  itself. 

M.  There  is  the  difference.  With  nitrogen  it  doesn't 
happen  by  itself.  I  see  you  ask  why  not.  The  answer  is 
that  hydrogen  and  oxygen  when  they  enter  into  combina- 
tion give  out  energy ;  indeed,  you  saw  how  much  heat  is  pro- 
duced by  their  combination.  But  to  make  nitrogen  combine, 
work  or  energy  must  be  spent.  And  since  work  is  never  a 
free  gift,  combined  nitrogen  has  a  much  higher  value  than 
free  nitrogen,  although  it  is  the  opposite  with  hydrogen. 

P.  But  not  with  oxygen? 

M.  Plants  make  free  oxygen;  we  shall  come  to  that 
later.  And  because  free  oxygen  can't  exist  in  plants, 
but  spreads  itself  through  the  air,  it  costs  nothing.  If 
oxygen  were  a  solid  or  a  liquid  substance  it  would  be 
collected  just  as  we  gather  grain  and  fruit,  and  would 
be  sold. 

P.  So  the  value  of  these  substances  does  not  lie  in 
themselves,  but  in  the  work  which  is  connected  with  them. 

M.  The  thought  is  right,  but  you  haven't  expressed  it 
correctly.  Different  substances  do  not  exist  without 
carrying  with  them  the  corresponding  amounts  of  work 
or  energy;  therefore  you  cannot  talk  of  them  without 
speaking  of  this  energy.  The  fact  is  this:  in  certain 
cases  the  uncombined  elements  contain  more  energy 
than  their  compounds;  in  other  cases,  as  with  nitrogen, 
the  opposite  is  the  case.    According  as  one  or  the  other 


1 86  CON  VERS ATlOm  ON  CHEMISTRY. 

relation  prevails,  the  elements  or  the  compounds  have 
the  higher  value. 

P.  But  their  value  consists  really  in  their  energy. 

M.  Yes,  that  is  right  on  the  whole. 

P.  How  does  it  happen  that  compounds  of  nitrogen 
are  important  for  war,  as  you  said,  because  gunpowder 
is  made  from  them?  Has  that  anything  to  do  with 
the  question  of  work? 

M.  Of  course.    A  gun  is  also  a  machine  for  doing  work. 

P.  No,  indeed!  It  is  used  for  destruction  and  not  for 
worL 

M.  What  you  call  destruction  is  also  work.  The 
object  is  to  give  to  the  ball  a  certain  high  velocity,  and 
in  order  to  do  that,  as  you  know  from  throwing  stones, 
a  good  deal  of  work  must  be  expended.  ♦ 

P.  Yes,  now  I  understand.  With  gas-engines,  of 
which  you  spoke  to  me  before,  an  explosion  is  also  used 
to  make  work. 

M.  Quite  right.  And  if  large  blocks  of  stone  or  of  ice 
have  to  be  got  rid  of  (and  that  involves  a  great  deal  of 
work),  they  are  blown  up  with  powder,  as  you  know. 
There  you  have  the  work  done  with  the  powder  before 
your  eyes. 

P.  Yes,  I  see  that.  But  what  has  it  all  to  do  with 
nitrogen  ? 

M.  Well,  in  compounds  of  nitrogen  there  is  more 
work  than  in  free  nitrogen,  and  so  these  compounds  can 
be  used  to  do  work. 

P,  Oh,  that  is  the  reason,  is  it? 

M.  Yes;  at  least  a  partial  reason. 

P.  Please  answer  this  question  that  I  wanted  to  ask 
before.  You  said  that  nitrogen  was  so  easily  produced 
from  its  compounds.  How  does  it  happen  that  there 
is  any  combined  nitrogen,  and  that  it  is  not  all  free? 


NITROGEN.  187 

M,  That  is  a  very  good  question.  The  answer  is 
that  by  many  kinds  of  work  which  are  available  in 
nature,  free  nitrogen  is  brought  into  combination.  For 
example,  many  plants,  like  peas,  beans,  lupins,  and 
vetches,  have  the  property  of  using  part  of  their  work 
in  causing  nitrogen  to  combine.  When  an  electrical 
discharge,  which  you  call  Hghtning,  passes  through  the 
air,  nitrogen  also  enters  into  combination.  Besides  that, 
people  are  very  careful  not  to  waste  combined  nitrogen. 
The  dung  of  animals  contains  a  large  quantity,  and  the 
farmer  spreads  it  on  his  fields,  where  it  is  absorbed  by 
plants. 

P.  So  that  is  why  they  spread  it  on  fields.  I  could 
never  see  why  that  nasty-smelling  stuff  could  do  any  good 
to  plants. 

M.  Besides  containing  combined  nitrogen,  manure  con- 
tains other  substances  which  plants  require,  but  nitrogen 
is  the  most  important  because  it  is  the  dearest.  More- 
over, though  manure  could  be  made  to  have  no  smell, 
it  would  not  help  us,  because  the  evil- smelling  substances 
contain  nitrogen,  which  would  be  lost  if  they  evaporated. 

P.  So  the  bad  smells  come  from  nitrogen? 

M.  A  good  many  do.  Do  you  know  the  smell  that 
wool  gives  off  in  burning? 

P.  Yes;  it  is  abominable. 

M.  Many  other  substances  give  a  similar  smell;  for 
example,  horn,  flesh,  leather,  and  feathers.  All  these 
substances  contain  nitrogen,  and  that  forms  a  means  of 
recognizing  them.  Wood  and  sugar  and  starch  also  give 
disagreeable  smells  when  they  burn,  but  they  haven't 
this  extremely  unpleasant  odor;  they  contain  no  nitrogen. 

P.  When  milk  boils  over  in  the  pan  it  smells  as  un- 
pleasant as  burnt  hair.    Does  it  contain  nitrogen  too  ? 


1 88  CONyERSATIONS  ON  CHEMISTRY. 

M.  Certainly;  casein,  which  is  contained  in  milk,  is 
a  compound  of  nitrogen. 

P.  Is  casein  contained  in  cheese? 

M.  Yes. 

P.  Old  cheese  has  a  different  kind  of  smell. 

M.  That  is  also  due  to  its  containing  compounds  of 
nitrogen. 

P.  Do  all  nitrogen  compounds  smell  bad  ? 

M.  Not  all,  but  most  of  them.  But  nitrogen  is  not 
the  only  element  that  has  this  unpleasant  property. 
Many  sulphur  compounds  have  a  disagreeable  smell, 
though  of  quite  a  different  character. 

23.  AIR. 

P.  You  told  me  yesterday  a  great  deal  about  the  com- 
pounds of  nitrogen,  but  you  didn't  show  me  a  single  one. 
There  must  be  hundreds  of  them. 

M.  That  is  quite  true.  You  will  have  to  wait  till 
later  to  learn  about  the  individual  compounds,  because 
they  exhibit  pretty  complicated  relationships.  For  the 
present  we  have  still  much  to  learn  about  free  nitrogen. 

P.  I  thought  there  wasn't  much  to  say  about  it;  you 
said  something  of  that  sort. 

M.  Yes,  so  far  as  concerns  its  properties  as  an  element. 
But  because  nitrogen  is  the  chief  constituent  of  the  air, 
we  must  take  air  as  our  subject.  Our  whole  life  and 
everything  that  we  do  takes  place  in  air,  and  so  we  must 
learn  about  its  properties  and  know  how  to  interpret  them 
rightly,  in  order  that  we  may  not  make  frequent  mistakes. 

P.  Yes,  no  one  can  live  without  air.  But  you  told  me  it 
was  the  oxygen  which  was  necessary  to  life,  and  that 
animals  are  suffocated  when  placed  in  nitrogen. 

M.  Quite  right;    but  we  will  not  discuss  that  again. 


AIR. 


189 


Air,  however,  is  a  gas;  it  is  the  best  known  and  most 
widely  spread  of  all  gases.  For  that  reason  we  will 
now  study  the  properties  of  gases,  taking  it  as  an  example. 

P.  I'm  glad  of  that,  for  I  must  say  that  gases  always 
strike  me  as  queer.  It  is  easy  to  see  and  grasp  solid  and 
liquid  things;  but  whether  a  flask  contains  hydrogen,  or 
oxygen,  or  ordinary  air,  I'm  sure  I  can't  tell.  For  all  I 
know  there  might  be  nothing  in  the  flask. 

M.  Yes,  I  quite  beheve  you;  for  as  gases  are  hardly 
ever  visible,  people  don't  generally  know  much  about 
them.  So  I  will  show  you  something.  You  know  that 
we  live  surrounded  by  gas,  by  air.  A  wind  or  a  storm  will 
teach  you  that  air  is  a  substance;  just  as  a  soHd  or  liquid 
body  in  motion  can  move,  throw  down,  and  break  other 
bodies,  so  also  can  air  in  motion. 

P.  Why  can't  we  see  air? 

M.  Because  we  are  surrounded  by  it.  Fishes  can't 
see  the  water  in  which  they  swim.  But  when  air  is 
surrounded  with  water  it  becomes 
visible.  I  blow  air  through  this 
tube  into  a  tall  glass  full  of  water. 
Now  you  can  see  little  quantities 
of  air,  as  round  bubbles,  quite  well 

(Fig.  39). 

P.  But    I    see    nothing    in    the 
bubbles  themselves. 

M.  Of  course  not,  because  air  is 
transparent.     You   see   nothing   in 
the  water  in  the  glass;    you  only 
recognize  the  surface  which  divides  the  water  from  the  air 
in  the  glass.     It  is  exactly  the  same  with  the  air-bubble. 

P.  But  I  can't  understand  in  the  least  how  you  can 
see  air  in  water  when  they  are  both  transparent. 


Fig.  39. 


190 


CONVERSATIONS  ON  CHEMISTRY. 


M.  It  is  true  they  are  both  transparent,  but  they  affect 
the  passage  of  Hght  in  different  ways.  In  physics  we  call 
that  the  difference  of  rejractivity.  For  that  reason,  too, 
you  see  no  particular  colour,  but  only  differences  between 
light  and  dark. — But  now  we  will  consider  the  air  from 
another  point  of  view.  In  your  lessons  in  physics  you 
have  learnt  something  about  the  pressure  of  the  atmos- 
phere and  about  the  barometer;  let  us  take  up  that 
now.     What  is  a  barometer? 

P.  A  tube  full  of  mercury,  closed  at  the  top  and  open 
below. 

M,  Yes,  that  is  pretty  correct.    I  have  here  a  glass  tube 

which  can  be  closed  above  by  a  glass  stop-cock.     At  the 

«  bottom  is  a  narrow  rubber  tube,  attached 

7^  by  its  other  end  to  a  second  tube  which  is 

open  (Fig.  40).     I  open  the  stop-cock  and 

pour  mercury  into  the  other  tube,  till  the 

rubber  tube  is  quite  full  and   the   glass 

tubes  are  half  filled  with  mercury.     I  fix 

each    tube   vertically   in  a   retort- stand; 

now,  what  is  the  level  of  the  mercury  ? 

P.  It  should  stand  at  the  same  height 

in  both  legs,  and  so  it  does. 

M.  And  now, when  I  raise  the  plain  tube  ? 

P.  The  mercury  will  rise  on  the  other 

side  too.    Take  care;  it's  running  through 

the  stop-cock. 

M.  I  will  shut  it,  then.     Now  I  will 

lower   the    other   tube    again.     But    the 

mercury  doesn't  follow  down;   it  remains 

close  up  to  the  top.     Why? 

P.  Because  the  top  is  shut.     No  ail 

Fig.  40.         can  enter. 


/ilR,  191 

M.  What  has  air  to  do  with  the  level  of  the  mercury  ? 

P.  I  suppose  it  has  something  to  do  with  the  pressure 
of  the  atmosphere.  Wait,  let  me  think.  Yes,  air  can 
press  on  the  mercury  in  the  open  tube,  but  not  in  the 
closed  one. 

M.  Quite  right.  But  now  the  mercury  begins  to 
sink  below  the  stop-cock.  That  isn't  because  the  top 
leaks,  for  when  I  raise  the  other  tube  the  mercury  rises 
again  to  the  stop-cock;  and  when  I  lower  it  the  mercury 
falls  again.     What  is  the  reason  of  that? 

P.  The  pressure  of  the  air  can't  keep  the  mercury 
up  any  longer. 

M.  That  is  so.  If  I  raise  the  open  tube  the  mercury 
rises  in  the  other  one,  and  when  I  lower  it,  it  falls.  Now 
we  will  take  some  measurements.  I  place  both  tubes 
close  together  and  measure  how  much  higher  one  column 
is  than  the  other.  It  is  75  centimeters.  If  I  now  move 
the  plain  tube  up  or  down  the  difference  is  always  75 
centimeters.  The  pressure  of  the  atmosphere  is  there- 
fore 75  centimeters  of  mercury. 

P.  Yes,  that  is  the  height  of  the  barometer.  Is  that 
the  same  as  30  inches? 

M.  Yes,  30  inches  is  nearly  equal  to  76  centimeters. 
But  the  pressure  of  the  atmosphere  is  a  pressure,  and 
75  centimeters  is  a  length.  How  can  you  express  a 
pressure  in  units  of  length? 

P.  The  pressure  of  a  liquid  depends  on  its  height. 

M.  Doesn't  it  depend  on  the  width  of  the  column  of 
liquid  ? 

P.  No,  I  learned  that  it  depends  only  on  its  height. 

M.  Yes,  for  any  one  liquid.  But  for  different  liquids 
the  pressure  depends  on  the  density  too.  Mercury  is 
13I  times  as  heavy  as  water,  and  so  for  an  equal  height 


192  CONVERSATIONS  ON  CHEMISTRY. 

it  presses  13 J  times  as  strongly.  If  you  wish  to  get  the 
same  pressure  with  water  as  with  mercury — 

P.  The  height  should  be  13 J  times  less. 

M.  That's  the  wrong  way  round.     Think  in  words. 

P.  Mercury  is  13 J  times  as  heavy  as  water,  and  so  it 
presses  13 J  times  stronger,  or  water  presses  13 J  times 
less  than  mercury — yes,  now  I  see,  the  column  of  water 
must  be  13 J  times  the  height  of  the  mercury. 

M.  That  is  right  now.  What  would  be  the  height  of 
a  water  barometer  then? 

P.  13 J  times  75  centimeters  is  1012J. 

M.  Yes,  more  than  10  meters.  Now  can  you  remember 
whether  the  pressure  of  the  atmosphere  always  remains 
the  same? 

P.  No,  it  changes;  in  fine  weather  the  barometer 
stands  high,  and  when  it  rains  it  stands  low. 

M.  Yes,  a  high  pressure  often  means  fine  weather, 
and  vice  versd;  but  this  isn't  always  the  case,  because  the 
atmospheric  pressure  is  affected  by  many  different  con- 
ditions; however,  we  won't  discuss  that  just  now.  You 
know  that  the  normal  height  of  the  barometer  is  30  inches, 
which  is  practically  the  same  as  76  cms.,  and  that 
pressure  is  used  as  a  standard  and  is  called  an  atmosphere. 
Can  you  give  the  meaning  of  the  word  atmosphere? 

P.  Yes,  air. 

M.  The  derivation  of  the  word  means  a  sphere  or 
ball  of  air.  It  refers  to  the  pressure  of  the  atmosphere. 
In  physics  the  pressure  is  usually  measured  in  centimeters 
of  mercury.  Now  pay  attention  to  this :  i  atm.  =  76  cms. 
mercury;  and  i  cm.  mercury  =  ^J^^-  atm.  But  to-day  the 
pressure  of  the  atmosphere  is  only  75  cms.,  that  is  ^f, 
or  0.987  atm.  When  I  open  the  tap  and  let  air  in  after 
I  have  raised  the  other  tube,  I  take  in  a  definite  quantity 


AIR.  193 

of  air,  and  I  know  that,  like  all  the  air  in  this  room,  it 
exerts  a  pressure  of  75  cms.  of  rhercury.  I  level  the 
mercury  so  that  it  stands  exactly  at  the  mark  100.  That 
means  that  the  tube  contains  exactly  100  cubic  centi- 
meters of  air.  Now  I  close  the  tap  again,  so  that  this 
quantity  of  air  can  alter  its  volume  only  when  the  level 
of  the  mercury  is  altered.  Now  the  apparatus  is  ready 
for  the  experiment. 

P.  What  are  you  going  to  do? 

M.  I  wish  to  show  you  how  the  volume  of  the  air 
changes  when  the  pressure  is  changed.  First  I  will 
lower  the  other  tube;  what  do  you  see? 

P.  The  mercury  in  the  tube  with  the  stop  cock  falls, 
but  much  less. 

M.  Now  we  will  measure  the  volume  of  the  air,  and 
the  pressure.  I  can  read  off  the  volume  from  the  division 
on  the  tube;  it  is  120  cc.  To  find  the  pressure  I  must 
measure  the  distance  between  the  two  mercury  columns; 
it  is  12.5  cms.    What  is  the  pressure  of  the  air  now? 

P.  12.5  cms.  of  mercury. 

M.  Wrong. 

P.  You  said  it  yourself. 

M.  I  said  that  the  difference  was  12.5  cms.  In  which 
tube  does  the  mercury  stand  higher? 

P.  In  the  tube  with  the  tap  where  the  air  is  contained. 
Yes,  the  pressure  must  be  less  there. 

M.  Less  than  what? 

P.  Than  it  was  to  begin  with. 

M.  Quite  right.     What  was  it  to  begin  with? 

P.  I  don't  know. 

M.  Yes,  you  do.  Just  think!  What  did  I  tell  you  at 
the  beginning  of  the  experiment  ?  What  was  the  pressure 
of  the  air  when  I  turned  the  stop  cock? 


1 94  CON  VERSA  TIONS  ON  CHE  MIS  TR  Y, 

P.  Oh,  now  I  remember,  it  was  the  pressure  of  the 
atmosphere,  75'  cms. 

M.  And  what  is  it  now? 

P.  12.5  cms.  less,  that  is  62.5  cms.     Is  that  right  now? 

M.  Yes.  Let  us  set  it  several  times  and  measure  the 
volume  and  the  pressure  of  the  air.  We  will  make  a 
tabk. 


Pressure, 

Volume. 

75       cms. 

mercury 

100  CCS. 

62.5    " 

(< 

120  " 

60 

(( 

150  " 

37-5    " 

(( 

200  " 

25       " 

(( 

300  ♦' 

P.  What  is  the  use  of  that? 

M.  I  want  to  show  you  how  to  discover  a  law  of  nature. 
We  have  two  quantities,  pressure  and  volume,  which 
change  with  each  other;  whenever  we  give  one  some 
definite  value,  the  other  must  also  have  a  definite  value 
which  we  cannot  control. 

P.  But  the  volume  depends  only  on  the  pressure;  I 
don't  see  how  the  pressure  depends  on  the  volume.  To 
get  a  definite  volume  we  must  alter  the  pressure. 

M.  That  depends  on  the  apparatus  we  are  using.  If 
you  were  closing  the  mouth  of  an  empty  bicycle  pump — • 
I  mean  one  filled  with  air — and  then  press  in  the  piston, 
you  alter  the  volume  of  the  air,  and  you  can  easily  feel 
that  the  pressure  rises,  because  it  is  more  diiB&cult  to  push 
in  the  piston. 

P.  Yes,  I  see  that. 

M.  Well,  then,  you  see  from  the  table  that  the  greater 
the  pressure  the  smaller  the  volume.  If  we  call  the 
pressure  p  and  the  volume  v  we  know  that  for  each  value 
of  p  there  is  a  definite  value  of  v. 

P.  What   is   the   law  for   this? 


AIR.  195 

M.  It  should  make  it  possible  to  calculate  for  each  p 
the  corresponding  v,  and  vice  versd. 

P.  How  can  that  be  done? 

M.  By  finding  a  method  of  calculating,  or,  as  it  is 
called,  a  formula,  by  means  of  which  the  one  value  can 
be  calculated  from  the  other. 

P.  I  don't  understand. 

M.  Suppose  you  have  10  apples;  some  in  your  pocket, 
the  others  in  your  hand.  If  we  call  /  the  number  of 
apples  in  your  pocket  and  h  those  in  your  hand,  if  you 
know  h  you  can  calculate  /,  and  if  you  know  t  you  can 
calculate  h.    How  is  this  possible? 

P.  Because  I  know  that  the  sum  of  both  is  10. 

M,  So  the  sum  of  t  and  h  is  equal  to  10,  and  the  formula 
is 

t+h=  10, 

From  this  formula  you  can  calculate  ^  if  /t  is  given,  or  h  if 
you  know  /. 

P.  That  is  very  nicely  put.  But  when  I  come  to  think 
of  it  it  is  quite  unnecessary,  because  I  know  it  without 
the  formula. 

if.  You  only  think  so  because  the  formula  is  so  simple 
and  the  process  so  common.  But  perhaps  we  can  bring 
our  measurements  of  the  pressure  and  volume  of  the 
air  into  a  similar  simple  formula. 

P.  Let  me  try.  75+100=157,  62.5+120=182.5, 
60+150  =  210.  No,  that  won't  do,  the  sum  always  gets 
bigger. 

M.  So  the  addition  formula  doesn't  work.  You  might 
have  seen  that  at  first.  For  you  can  only  add  similar 
quantities,  like  apples  to  apples,  but  not  different  quan- 
tities like  pressures  and  volumes. 


196  CONVERSATIONS  ON  CHEMISTRY, 

P.  What  sort  of  formula  can  it  possibly  be? 

M.  If  p  grows  larger,  v  grows  smaller.  What  com- 
bination of  p  and  V  has  this  property? 

P.  A  great  many,  no  doubt. 

M.  Of  course,  but  not  many  simple  ones.  Try  to 
find  the  simplest  possible  one. 

P.  Perhaps  the  product  ?  If  one  factor  becomes  larger 
the  other  must  grow  smaller  to  get  the  same  product. 

M.  Try  if  that  works. 

P.  75X100=7500,  62.5X120  =  7500,  50X150  =  7500, 
37.5X200=7500,  25X300=7500.  Upon  my  word  it's 
right! 

M.  Then  write  the  formula. 

P.  pXv^JS^' 

M.  Right.  Now  you  have  found  the  law  which  con- 
nects the  pressure  and  volume  of  the  air  with  each  other, 
or  makes  them  dependent  upon  each  other. 

P.  I  should  never  have  found  that  out  without  your 
help. 

M.  I  quite  agree. 

P.  Tell  me,  did  you  find  it  out  by  yourself  ? 

M.  No.  An  English  physicist  named  Boyle  dis- 
covered it  nearly  two  hundred  and  fifty  years  ago,  and 
it  goes  by  the  name  of  Boyle's  law.  But  we  haven't 
quite  grasped  the  law  yet.  Supposing  the  pressures  had 
not  been  given  in  centimeters  of  mercury,  but  in  atmos- 
pheres, all  the  values  of  p  would  have  been  76  times  less. 
Then  the  product  py.'v  would  not  have  been  equal  to 
7500,  but  '^^776  =  98.7?  and  the  formula  would  have  been 

P.  I  see  that. 

M.  Further,  if  I  had  not  had  100  cc.  of  air,  but  only 
80— 


^IR.  197 

P.  Then  the  product  would  have  been  75X80  =  6000. 

M.  Yes,  the  first  one  would  be.  But  what  about  the 
others  ? 

P.  That  can't  be  told  beforehand. 

M.  Oh,  yes,  it  could;  only  think.  I  had  taken  ^Yioo 
or  Ys  of  the  original  quantity  or  air.  Whatever  I  do 
with  the  air,  this  quantity  always  remains  Yg  of  the 
original.  And  therefore  its  volume  remains  under  all 
circumstances  Ys  of  its  original  volume.  Hence  all  the 
figures  for  v  would  be  decreased  in  the  same  proportion. 

P.  Wouldn't  the  values  of  p  be  also  proportionately 
decreased  ? 

M.  No.  The  pressure  is  equally  distributed  through 
the  whole  quantity  of  air,  and  so  it  doesn't  matter  whether 
you  take  a  larger  or  a  smaller  fraction  of  it.  The  100  c.c. 
with  which  the  experiment  was  made  were,  of  course, 
only  an  arbitrary  amount  of  the  whole  of  the  air  in  the 
room,  which  had  everywhere  the  pressure  of  75  cms. 

P.  Why  do  the  pressures  behave  differently  from  the 
volumes  ? 

M.  As  I  have  often  said  to  you  in  such  cases  you 
mustn't  ask  why,  but  notice  that  certain  quantities  behave 
in  one  way  and  others  in  another.  Temperatures  be- 
have like  pressures.  For  instance,  if  a  mass  of  water 
has  a  definite  temperature  every  part  of  the  mass  has 
the  same  temperature  whether  it  is  large  or  small. 

P.  But  surely  a  quantity  of  water  can  have  different 
temperatures  at  different  places 

M.  Quite  true,  but  I  was  speaking  of  masses  which 
have  the  same  temperature  all  through.  You  see  the 
similarity  here  between  temperature  and  pressure.  If 
they  have  different  values  at  different  parts  of  a  continu- 
ous mass  they  don't  remain  in  that  condition,  but  become 


ipS  CONVERSATIONS  ON  CHEMISTRY. 

equal.  But  we  must  go  back  to  our  experiments.  You 
have  seen  there  is  something  arbitrary  about  the  number 
7500,  since  it  depends  on  the  quantity  of  air  taken  and 
on  the  units  in  which  the  pressure  was  measured.  We 
must  give  our  formula  such  a  form  as  not  to  contain 
any  arbitrary  units.    Hence  we  write  Boyle's  law  thus; 

pv=C. 

P.  What  does  C  mean? 

M.  It  means  that  the  product  pv  has  a  definite  value 
C,  which  remains  unchanged  as  long  as  only  the  values 
of  p  and  V  change.  For  this  reason  p  and  v  are  called 
variables,  while  C  is  a  constant;  that  is,  an  invariable,  or 
an  unchangeable  number. 

P.  But  C  can  have  different  values  too. 

M.  Only  when  you  change  the  quantity  of  air  taken. 

You  have  already  seen   that   the   product  pv  increases 

or  diminishes  its  value  in  proportion   to  the  quantity 

taken.     If  you  call  q  the  quantity,  you  can  write  C  =  qKy 

where  K  is  another  constant  which  is  no  longer  dependent 

on  the  quantity  q.    Place  this  value  of  C  in  the  equation 

and  it  becomes 

iru 
pv=qK    or    —=K, 

P.  What's  the  use  of  this  formula? 

M.  It  makes  it  possible  to  apply  the  law  to  any  quan- 
tities of  gas.  If  the  amount  in  cubic  centimeters  is 
measured  at  75  cm.  pressure,  our  former  constant  C  =  7500 
would  be  written:  75oo  =  iooi<C  or  K=yS'  If  the  num- 
ber 75  be  introduced  into  the  last  equation,  then 

pv 


AIR.  199 

And  this  equation  holds  for  all  experiments  with  any 
quantity  of  air  you  like. 

P.  I  should  like  to  see  that. 

M.  We  will  make  an  experiment.  I  shut  off  60  c.c. 
of  air,  at  atmospheric  pressure,  lower  the  other  tube  till 
the  volume  of  the  air  is  100  c.c.  What  is  the  pressure 
now? 

P.  How  can  I  tell? 

M.  You  ought  to  be  able  to  tell;  it  follows  from  the 

formula.     You   have   only   to   introduce    the   values   in 

order  to  calculate  p.    You  know  the  volume  v=ioo  and 

the  quantity  q  =  6o. 

_.    ^Xioo  ^  . 

F.  — T =  75j  so  />  =  45;  the  pressure  is  45  cms. 

M.  Now,  how  do  I  get  the  pressure  45  cms.? 

P.  Stop;  I  can  calculate  that.  The  pressure  of  the 
atmosphere  is  75  cms.,  and  75—45  =  30,  so  the  mercury 
in  the  open  tube  must  stand  30  cms.  below  its  level  in 
the  other.     Let  me  measure  it.     It  agrees  exactly. 

M.  Are  you  surprised? 

P.  Yes;  it  is  almost  like  magic. 

M.  What  is? 

P.  That  you  can  tell  such  a  thing  beforehand. 

M.  That  is  the  use  of  laws  of  nature  to  enable  us  to 
foretell  what  will  happen  in  the  future.  Only  think  of 
the  predictions  of  the  eclipses  of  the  sun  and  moon. 

P.  Yes,  IVe  taken  it  all  in,  but  I  can't  say  that  I'm 
accustomed  to  it  yet. 

M.  That  is  quite  to  be  expected;  but  we  shall  often 
have  to  do  with  similar  things,  and  you  will  soon  become 
famihar  with  such  ideas. 


200  COI^yERSATIONS  ON  CHEMISTRY, 


24.  CONTINUITY  AND  EXACTNESS. 

M.  Have  you  understood  all  I  told  you  about  Boyle's 
law? 

P.  Yes,  I  understood  all  that  you  said.  But  I  couldn't 
understand  a  great  deal  of  what  you  didn't  say. 

M.  Well,  then,  ask  questions. 

P.  Yesterday  we  found  the  pressures  corresponding  to 
five  or  six  different  volumes.  And  you  got  out  the 
formula  ^=7500,  which  fitted  these  few  cases,  and 
then  you  used  them  for  quite  different  cases.  How  did 
you  do  that? 

M.  It's  a  very  sensible  question,  and  I  will  try  to  make 
it  clear  to  you.  When  you  blow  several  times  into  a 
toy  trumpet,  you  always  hear  the  same  tone;  you  will 
expect  always  to  hear  the  same  tone  in  future  whenever 
you  blow  the  trumpet. 

P.  Of  course. 

M,  It  is  much  the  same  with  the  formula.  When- 
ever you  multiplied  pressure  and  volume  together  the 
product  was  7500,  and  hence  I  had  the  right  to  expect 
that  it  would  always  be  the  same.  You  may  remember 
that  our  expectation  was  not  disappointed.  We  tested 
the  formula,  and  found  it  held. 

P.  Ohl  I  didn't  think  it  was  as  simple  as  that. 

M.  Nor  is  it.  It  is  an  instance  of  a  very  important 
law,  one  which  is  so  general  and  accepted  that  we  always 
use  it. 

P.  A  general  law!     I  don't  know  of  any  in  this  case. 

M.  Certainly  you  do,  because  you  constantly  use  it. 
The  fact  is  that  you  are  not  in  the  habit  of  expressing 


CONTINUITY  AND  EXACTNESS.  20I 

It  in  the  form  of  a  law.  It  is  the  law  of  persistence 
oj  natural  phenomena. 

P.  How  is  the  law  expressed,  then? 

M.  If  an  event  takes  place  under  definite  conditions  it 
will  always  take  place  in  future  ij  the  conditions  are  the 
same. 

P.  But  that  goes  without  saying. 

M.  People  always  say  that  of  things  they  haven't 
thought  about.  You  had  just  put  a  question  which  was 
answered  by  the  law. 

P.  Yes,  but  that  was  a  case  that  was  new  to  me. 

M.  It  was  only  a  new  application  of  the  general  law; 
it  was  no  new  law.  You  see  at  once  how  very  import- 
ant it  is  to  express  definitely  such  self-evident  laws.  If 
you  had  known  how  to  express  this  law  before,  you  could 
have  answered  your  own  question. 

P.  So  I  will  in  future. — But  what  we  have  just  been 
talking  about  wasn't  all  that  I  wanted  to  know.  Of 
course,  I  believe  that  if  we  were  repeating  exactly  the 
same  experiment  with  the  same  volumes,  we  should  get 
the  same  pressures.  But  there  are  many  other  pressures 
and  volumes  between  those  that  we  didn't  measure- 
How  can  we  know  that  the  formula  will  suit  these,  for 
the  conditions  are  no  longer  the  same  ? 

M.  That  is  a  very  good  question.  It  is  also  answered 
by  a  law  of  nature. 

P.  Another! 

M.  You  are  getting  tired  of  laws  of  nature,  are'nt 
you?  Don't  be  frightened;  it  is  another  self-evident 
one. 

P.  All  that  I  mean  is  that  we  will  soon  have  so  many 
that  there  won't  be  another  left  to  be  discovered. 

M.  So  much  the  better. 


202  CONVERSATIONS  ON  CHEMISTRY. 

P.  So  much  the  better? 

M.  Laws  of  nature  tell  us  what  we  have  to  ex- 
pect under  definite  conditions.  Now  no  law  includes 
all  such  conditions,  but  only  one  or  a  few.  Hence, 
in  order  to  know  exactly  what  will  be  the  final 
result,  we  must  know  the  laws  for  all  possible 
conditions,  so  that  all  uncertainty  disappears  and 
only  one  thing  is  possible.  Then,  that  particular  result 
will  really  take  place. 

P.  Oh !  so  what  you  really  meant  was  that  there  should 
be  no  doubt  of  the  result. 

M.  That  wasn't  quite  your  view,  was  it?  The  general 
law  of  which  I  spoke  is  one  that  relates  to  the  continuity 
of  natural  phenomena. 

P.  Explain  that. 

M.  We  have  just  seen  that  we  can  express  natural 
laws  in  the  form : — if  this  is  the  case  the  other  will  follow. 
Now  the  "this"  is  often  not  one  single  definite  thing, 
but  something  capable  of  different  degrees,  shades,  and 
magnitudes,  and  other  things  depending  on  these.  If 
we  alter  one  of  these  continuously,  that  means,  so  that 
there  is  never  a  sudden  change  in  its  value,  the  other  also 
alters  continuously,  and  its  value  makes  no  sudden 
change,  either. 

P.  I  think  that  I  can  remember  a  Latin  proverb 
about  that.  Natura  non  jacit  saltus  Nature  makes 
no  jumps. 

M.  Yes,  but  it's  only  a  half-truth.  Nature  does 
make  jumps;  but  then  all  magnitudes  which  are 
connected  with  each  other  make  jumps  at  the  same 
time. 

P.  I  can't  think  of  an  example. 

M.  Just  think  of  the  transition  from  ice  to  water. 


CONTINUITY  AND  EXACTNESS.  203 

When  the  solid  changes  to  the  Hquid  its  physical  state 
changes  suddenly,  and  at  the  same  time  its  volume  sud- 
denly becomes  one- eleventh  smaller,  and  its  pow^er  of 
refracting  light,  its  electrical  properties,  and  countless 
other  things  suddenly  change  their  value  too. 

P.  Do  all  its  properties  change  at  the  same  time? 

M.  Nearly  all;  mass  and  v^eight,  however,  remain  un- 
changed. 

P.  I  still  don't  see  vv^hat  all  this  has  to  do  with  my 
question. 

M.  You  asked  how  it  was  possible  from  a  few  values 
of  pressure  and  volume*  for  which  you  found  the  product 
constant,  to  assume  all  intermediate  values  to  be  the 
same.  But  it  can  be  deduced  from  the  law  of  continuity. 
For  if  the  product  is  the  same  for  any  two  values  of 
pressure  which  lie  somewhat  near  each  other,  then  it 
must  also  be  the  same  for  the  pressures  which  lie  between 
these  two,  unless  one  of  the  factors  suddenly  alters  its 
value.  But  the  law  of  continuity  excludes  such  a  pos- 
sibihty. 

P.  I  don't  quite  understand  that. 

M.  Think  of  the  example  of  the  toy  trumpet.  If  you 
blow  gently  the  first  time  and  hard  the  second,  and  the 
tone  is  always  the  same  height,  you  might  conclude  that 
if  you  were  to  blow  only  fairly  hard,  you  would  still  hear 
the  same  tone. 

P.  Yes,  of  course. 

M.  You  have  just  appHed  the  law  of  continuity. 

P.  Oh,  is  it  as  simple  as  all  that? 

M.  It  is;  the  difficulty  isn't  in  understanding  the  law, 
but  in  applying  it  to  unfamiHar  cases.  But  now  we  will 
go  on  with  Boyle's  law.  Up  to  the  present  we  have  only 
tested  it  for  pressures  which  lie  below  that  of  the  atmos- 


204  CONVERSATIONS  ON  CHEMISTRY. 

phere.  What  do  you  think — will  it  hold  for  higher  pres- 
sures ? 

P.  I  don't  know  any  reason  either  for  or  against. 

M.  Yes,  you  know  one  in  favoui  of  it:  the  law  of 
continuity.     Try  to  apply  it. 

P.  At  pressures  which  are  a  little  higher  than  one 
atmosphere  the  product  pv  will  still  be  the  same. 

M.  Quite  right. 

P.  But  how  far  can  I  go? 

M.  You  can  only  tell  by  experiment.  We  will  lift  up 
the  open  tube  as  far  as  we  can.  Now  the  volume  has 
contracted  to  40  c.c,  and  the  difference  of  the  mercury 
level  is  more  than  one  metre.  I  will  use  a  second  metre 
rule;   now  I  find  112 J  cms.     Does  that  agree? 

P.  112^X40  =  4500.  No,  the  product  is  much  too 
small. 

M.  Now  just  think. 

P.  Of  course  I  forgot  the  pressure  of  the  atmosphere. 
But  I  can't  subtract  112 J  from  75 

M.  Why  should  you? 

P.  Because — no,  how  stupid  I  was;  the  mercury  is  now 
pressing  on  thfe  same  side  as  the  atmosphere,  so  I  must 
add  them  and  not  subtract:  ii2j+75  =  187J,  and 
1871X40  =  7500.     It's  quite  correct. 

M.  What  conclusion  would  you  draw  for  pressures 
which  lie  between  that  and  one  atmosphere? 

P.  The  product  will  agree  for  those  too,  according 
to  the  law  of  continuity. 

M.  You  needn't  laugh,  it's  quite  right.  To  convince 
yourself  you  may  make  some  measurements  with  the  ap- 
paratus. 

P.  That's  capital;  thank  you  very  much. 

M.  Only  take  care  that  you  don't  spill  any  mercur)^; 


CONTINUITY  /IND  EXACTNESS,  205 

you  had  better  take  the  lid  of  a  large  pasteboard  box  and 
work  with  your  apparatus  on  it. 


M.  Well,  have  your  measurements  been  successful? 

P.  Not  very.  The  product  of  pressure  and  volume 
didn't  always  come  to  7500;  it  was  sometimes  a  little 
more,  and  sometimes  a  little  less. 

M.  That  is  what  you  might  have  expected. 

P.  Then  is  Boyle's  law  not  exact  ? 

M.  The  law  is,  but  not  your  measurements.  How 
accurately  did  you  read  the  levels  ? 

P.  Oh,  it  wasn't  easy  to  hold  the  rule  upright  and  to 
read  the  level  of  the  mercury. 

M.  Look  here^  you  can't  have  been  a  whole  centi- 
metre wrong,  though  you  may  have  made  a  mistake  of 
some  millimetres.  Look  at  my  last  result,  where  the 
volume  was  40  c.c.  and  the  pressure  187J  cms.  If  I 
had  read  J  a  cm.  too  much  (which  was  quite  possible 
because  I  had  to  lengthen  the  rule)  I  should  have  got 
188X40  =  7520  instead  of  7500.  If  I  had  read  J  cm. 
too  little  the  product  could  have,  been  7480.  That  shows 
the  influence  of  experimental  error  on  the  result. 

P.  Yes,  my  numbers  were  something  like  these. 

M.  Moreover,  you  may  have  made  a  mistake  in 
measuring  your  volume.  The  tube  is  graduated  in 
cubic  centimetres  and  tenths,  and  you  may  have  made 
the  mistake  of  a  tenth.  If  you  had  read  40.1  instead  of 
40  it  would  have  given  1871X40.1  =  7518.75,  again  an 
erroneous  result.  If  you  had  also  read  the  pressure 
wrongly,   say    188  cms.,   the  product  would  have  been 

7538-8. 


2o6  CONIFERS  A  TIONS  ON  CHEMISTRY. 

P.  Well,  how  can  you  know  which  is  the  right 
number  ? 

M.  You  can  never  know,  for  every  experiment  con- 
tains an  error. 

P.  Even  when  you  measure  very  accurately  ? 

M.  Then  you  will  make  your  error  smaller,  but  it  will 
never  disappear. 

P.  But  is  nothing  ever  measured  accurately? 

M.  No  magnitude  is  ever  measured  so  accurately  that 
there  is  no  error  at  all.  There  are  only  measurements 
of  greater  or  lesser  accuracy. 

P.  But  what  can  be  done  if  the  numbers  are  as  differ- 
ent as  those  I  got?  What  value  should  be  taken  as  the 
correct  one? 

M.  You  can't  state  the  correct  value,  but  you 
can  state  one  which  is  probably  the  nearest  to  the  cor- 
rect one. 

P.  How  can  that  be  done? 

M.  Think  for  a  minute.  You  may  have  made  errors 
which  would  have  made  the  result  too  large,  as  well  a 
too  small,  and  so  the  correct  value  will  lie  somewhere  in 
the  middle,  between  the  largest  and  the  smallest  values 
which  you  found. 

P.  I  see  that. 

M.  Then  you  must  take  the  mean  value  of  all  your 
observations.  That  can  be  done  by  adding  all  the 
observed  values  together,  and  dividing  by  the  number 
of  observations.  The  quotient  is  called  the  mean  value, 
and  it  is  the  most  probable  one. 

P.  Please  let  me  try  so  that  I  may  understand  it. 
I  found  for  the  product  pv  the  numbers   7520,    7475, 

7492,  7533>  7506,  7491- 
M.  There  are  six  values;  add  them  and  divide  by  six. 


CONTINUITY  AND  EXACTNESS.  207 

P.   7520 

7475 
7492    . 

7533 
7506 
7491 

45017;  =7502.833.  .  .  .  How    many    decimals 

shall  I  write? 

M.  None  at  all. 

P.  But  then  I  shall  make  a  mistake. 

M'.  You  know  that  all  your  measurements  con- 
tain an  error.  If  you  examine  your  numbers  you  will 
see  that  even  the  tens  don't  agree,  and  the  units  must 
be  quite  uncertain.  Of  your  mean  value  7502.833  .  .  . 
the  o  in  the  tens  place  is  perhaps  right,  but  the 
two  units  are  quite  uncertain,  because  they  would  have 
come  out  differently  if  you  had  made  another  measure- 
ment. 

P.  So  I  will. — It  comes  out  751 1. 

M,  Do   the   calculation   with   the   seven   values  now; 

what  do  you  find? 

^    52528 

P'  '-^  =  7504. 

M.  You  see  you  have  got  two  more  units.  It 
would  lead  to  error  if  you  were  to  write  units  or  deci- 
mals. The  usual  plan  is  simply  to  write  o  for  such 
uncertain  places  to  show  that  you  can  make  no  statement 
about  them.     Do  that ;   what  is  your  mean  value  ? 

P.  7500. 

M.  Quite  right. — Now  let  us  go  back  to  the  question 
whether    Boyle's    law    holds    for    any    pressure.      The 


2o8  CONyERSATIONS  ON  CHEMISTRY. 

answer  is  that  it  has  been  shown  to  be  nearly  correct 
for  the  smallest  pressures  which  can  be  measured.  On 
the  other  hand,  at  higher  pressures  there  are  deviations, 
small  indeed  at  lo  atmospheres,  considerable  at  loo, 
and  very  great  at  looo. 

P.  Where  do  these  deviations  begin? 

M.  The  answer  depends  upon  the  exactness  of  the 
measurements.  The  more  exactly  the  pressures  and 
volumes  are  measured,  the  smaller  are  the  pressures  at 
which  the  first  deviations  can  be  detected. 

P.  So  Boyle's  law  is  not  exact. 

M.  No;  that  can't  be  said  of  any  law  of  nature. 
But  so  far  as  its  apphcation  is  concerned,  it  is  exact 
enough,  because  the  errors  of  our  measurements  will 
always  be  much  greater  than  the  deviations  from  the 
law. 


25.  THE  EXPANSION  OF  AIR  BY  HEAT. 

M,  Do  you  quite  understand  Boyle's  law  now? 

P.  I  think  I  do,  but  there  is  something  I  am  not  clear 
about.  You  once  told  me  that  air  expands  when  it  is 
heated.  But  then  the  same  quantity  of  air  at  the  same 
pressure  would  have  different  volumes;  it  would  have  a 
bigger  one  if  it  was  warm  and  a  smaller  one  if  it  was 
cold. 

M.  You  are  quite  right.  Boyle's  law  holds  only  for 
an  unchanging  temperature. 

P.  What  temperature? 

M.  For  any  temperature,  but  it  must  remain  constant. 
Our  experiments  were  carried  out  at  the  temperature  of 


THE  EXPANSION  OF  AIR  BY  HEAT.  209 

the  room,  which  was  about  18°.  If  it  had  changed 
much  while  we  were  making  them  our  results  would  not 
have  been  concordant. 

P.  Does  that  mean  that  Boyle's  law  is  not  worth  much  ? 

M.  It  has  lost  none  of  its  value,  only  you  have  learnt 
one  of  the  conditions  which  must  be  fulfilled  when  it  is 
appHed. 

P.  If  the  temperature  doesn't  remain  the  same,  what 
can  we  do? 

M.  Then  we  try  to  find  out  a  law  which  will  allow 
for  its  influence. 

P.  How  can  that  be  done? 

M.  If  we  know  how  much  the  volume  of  a  given  quantity 
of  gas  alters  when  the  temperature  is  changed  by  a  known 
amount,  we  can  apply  a  correction  to  our  measurements, 
so  that  the  result  comes  out  as  if  it  had  been  obtained 
at  some  one  definite  temperature. 

Pj  I  have  a  sort  of  general  idea,  but  am  not  clear  about 
it  yet. 

M.  You  will  soon  understand  it.  Here  is  a  narrow 
glass  tube  about  2  mms.  wide,  about  half  a  metre  long, 
closed  at  one  end,  and  containing  at  about  the  middle  a 
drop  of  mercury,  which  shuts  off  a  definite  quantity  of 
air.  If  I  warm  this  air  with  my  hand,  the  drop  moves 
forward,  and  it  goes  back  again  when  the  air  cools. 
Thus  you  can  see  and  measure  the  expansion  of  the  air 
by  heat. 

P.  It  is  just  like  a  thermometer. 

M.  Yes,  an  air-thermometer.  Now  I  place  the  tube 
in  melted  ice  and  mark  where  the  drop  stands  with  a 
small  india-rubber  ring. 

P.  Where  did  you  get  it? 

M.  I  cut  it  off  a  piece  of  rubber  tube  with  a  pair  A)f 


2IO 


CONVERSATIONS  ON  CHEMISTRY. 


scissors.  Now  I  measure  the  length  of  the  column  of 
air  which  is  cooled  to  o°  in  the 
ice;  its  length  is  273  mms. 
Now  I  will  heat  the  same  quan- 
tity of  air  to  100°,  the  boihng- 
point  of  water.  To  do  this  I 
fit  a  flask  with  a  somewhat  wide 
glass  tube  by  means  of  a  cork, 
and  boil  the  water  in  the  flask 
(Fig.  41).  When  I  place  the  tube 
in  the  steam  the  drop  moves  up 
considerably. 

P.  How  can  you  make  a  mark 

on  it  without  burning  your  fingers  ? 

M.  I    push    down    the    second 

india-rubber    ring  with  a  rod  to 

the  right  place.     Now  it  is  done. 

I  take  the  tube  out  and  measure 

the  length  of  the  column  of  air. 

The   second   ring   stands   at   373 

mms. 

P.  That  is  exactly  100  mms.  more;    a  millimetre  for 

each  degree.     How  does  that  come  out  so  exactly? 

M.  I  knew  beforehand  that  273  volumes  of  air  when 
heated  from  the  freezing-  to  the  boiling-point  of  water 
would  expand  exactly  100  volumes,  and  I  enclosed  exactly 
the  right  quantity  of  air  to  begin  with. 
P.  Did  you  do  that  at  0°  or  100°  ? 
M.  No;  I  looked  at  the  thermometer  in  the  room  and 
saw  that  it  stood  at  18°.  As  273  divisions  at  0°  expand, 
one  division  for  each  degree,  I  knew  they  would  occupy 
at  18°  273-1-18  =  291  divisions.  So  that  I  placed  the 
drop  of  mercury  291  mms.  from  the  end  of  the  tube. 


Fig.  41. 


THE  EXPANSION  OF  AIR  BY  HEAT.  211 

P.  How  did  you  do  that?  The  drop  doesn't  move 
when  I  tilt  the  tube. 

M.  That  is  very  simple;  it  doesn't  move  because  the 
air  can't  pass  the  drop.  I  pushed  a  horsehair  into  the 
tube  through  the  drop  and  it  moves  quite  easily  now. 
Look! 

P.  That's  neat.  But  how  did  the  air  pass?  Oh,  I 
see;  the  mercury  doesn't  quite  touch  the  glass  where  the, 
hair  is. 

M.  Yes;  what  is  called  surface-tension  makes  the  mer- 
cury curved,  and  the  air  can  not  pass  except  through  the 
narrow  groove  between  the  glass  and  the  hair.  But  we 
will  go  back  to  our  experiment.  We  will  make  a 
diagram  (Fig.  42).  The  horizontal  line  stands  for  a 
thermometer.  At  the  mark  o  water  freezes,  at  100  it 
boils.*     Each  millimetre  stands  for  one  degree. 

P.  I  understand  that. 

M.  Now  we  will  draw  horizontal  lines  each  of  which  rep- 
resents the  volume  of  the  air  in  our  experiment.  We  will 
make  the  horizontal  line  at  o,  273  mms.  long;  and  at  100; 
373  mms.     We  join  the  two  end-points  by  a  straight  line. 

P.  What  is  the  use  of  the  figure? 

M.  It  makes  it  possible  to  find  the  volume  of  the  air 
for  any  intermediate  temperature.  Pick  out  the  point 
18°  on  the  thermometer-line  and  measure  how  long  the 
horizontal  line  is. 

P.  It  is  290 — no,  291  mms.  long.  I  have  just  had  this 
number — yes,  it  was  the  place  where  you  set  the  mercury 
with  the  horsehair. 

M.  Yes,  that  is  the  volume  of  the  air  at  18°. 

P.  How  does  it  happen  that  the  right  number  comes? 

*  Fig.  42  is  reduced  to  a  quarter  of  its  right  size. 


212 


CONVERSATIONS  ON  CHEMISTRY, 


M.  That  is  very  simple;  for  each  degree  the  length  of 
the  air-line  increases  by  i  mm.,  and  so  the  ends  of  all 
these  Hnes  lie  in  a  straight  line. 

P.  Yes,  I  understand  that.    I  see,  too,  that  I  can  find 


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Fig.  42. 

out  what  the  volume  of  the  air  v^ill  become  when  it  has 
exactly  the  volume  273  divisions  at  0°.     But — 
M.  Well? 


THE  EXPAhlSlON  OF  AIR  BY  HEAT.  213 

P.  I  was  going  to  ask  something  stupid.  If  I  know 
it  for  273  divisions  I  can  calculate  it  for  any  other  number 
by  simple  proportion. 

M.  Quite  right.  When  273  divisions  measured  at  0° 
increase  one  division  for  each  degree,  one  division  will 
increase  by  Y273)  3,nd  for  any  number  of  degrees,  which 

may  be  called  /,  by  divisions.     You  can  draw  this 

^273 

more  easily  if  you  use  paper  ruled  in  millimetres.  There 
is  a  net  of  lines  on  the  paper  which  stand  exactly  i  milli- 
metre apart,  and  you  do  not  need  to  measure,  but  you 
can  count  the  numbers  directly. 

P.  But  I  must  count  the  lines. 

M,  That  is  very  easy;  for  every  fifth  and  tenth  line 
is  a  little  broader,  and  you  need  only  write  10,  20,  30, 
etc.,  on  these  lines. 

P.  Yes,  that  works  very  well. 

M.  Now  tell  me:  what  would  be  the  effect  of  cooling 
the  gas  below  o°? 

P.  I  think  that  its  volume  would  decrease  by  V273  for 
each  degree. 

M.  Quite  right.  In  our  drawing  you  need  only  pro- 
duce the  line  downwards  towards  the  left  in  order  to 
represent  volumes  below  0°. 

P.  But  I  don't  see  what  that  means;  the  line  gets 
nearer  and  nearer  the  thermometer-line  and  finally 
touches  it.  That  must  mean  that  the  volume  of  the 
gas  becomes  nothing,  and  if  I  produce  it  further,  less 
than  nothing. 

M.  Quite  right.  At  what  temperature  does  that 
happen  ? 

P.  About  -273°. 


214  CONVERSATIONS  ON  CHEMISTRY. 

M.  Yes;  if  air  loses  Vgys  of  its  volume  for  each  degree, 
at  273°  below  zero  nothing  is  left. 

P.  Is  that  really  the  case? 

M.  I  don't  know,  for  the  temperature  —273°  has 
never  been  reached. 

P.  Why  not? 

M.  It  hasn't  been  found  possible.  The  lowest  tem- 
perature ever  obtained  is  about  —260°,  and  from  the 
trouble  which  it  has  cost  to  lower  the  temperature,  we 
must  conclude  that  it  will  be  long  before  any  attempt 
to  lower  the  temperature  10°  more  will  be  successful. 

P.  Has  the  air  at  —  260°  really  the  small  volume  that 
is  shown  on  the  diagram? 

M.  The  volume  is  still  smaller,  but  the  reason  is  that 
at  —  190°  air  is  no  longer  a  gas,  but  condenses  to  a  liquid. 

P.  Oh,  then  this  part  of  the  diagram  has  no  meaning. 

M.  Yes  it  has.  There  are  other  gases,  for  example, 
helium,  which  behave  exactly  as  the  diagram  would  in- 
dicate, at  the  lowest  temperatures.  Helium  has  never 
been  liquefied;  at  lower  temperatures  it  would  be,  no 
doubt ;  but  we  can  imagine  a  gas  that  wouldn't  and  then  it 
would  behave  as  is  shown  in  the  diagram. 

P.  Does  the  diagram  apply  to  all  gases  ? 

M.  Yes,  all  gases  behave  exactly  Hke  air,  and  alter 
their  volume  by  Ysra  of  the  volume  which  they  occupy  at 
0°,  for  each  change  of  one  degree.  Here  we  again  have 
a  general  law  of  nature,  which  makes  it  possible  to  pre- 
dict the  behaviour  of  very  different  substances.  You 
can  say  beforehand  that  if  any  substance  is  a  gas,  it  will 
expand  by  Yava  of  its  volume  for  each  degree. 

P.  That  is  very  convenient. 

M.  You  can  see  that  all  gases  point  to  the  temperature 
—  273°  as  a  limiting  temperature.     It  is  probable  that  it 


THE  EXPANSION  OF  AIR  BY  HEAT.  2l5 

would  be  impossible  to  obtain  lower  temperatures  than 
—  273°.  This  would  therefore  be  the  lowest  possible  tem- 
perature; you  may  remember  we  spoke  of  this  before; 
and  if  we  mark  the  position  of  the  mercury  in  the  ther- 
mometer at  the  melting-point  of  ice,  with  the  number 
273°,and  the  boiling-point  of  water  with  373°,  we  should 
probably  never  be  obHged  to  calculate  with  negative 
temperatures.  We  therefore  call  the  temperature  —  2  73° 
the  absolute  zero,  and  the  temperatures  counted  up- 
wards from  it  are  called  absolute  temperatures. 

P  What  is  the  good  of  that? 

M,  It  is  very  useful  in  many  respects;  chiefly  in  the 
theory  of  heat,  and  the  time  hasn't  come  to  explain  that 
to  you.  But  I  will  tell  you  one  thing.  If  the  melting- 
point  of  ice  is  made  273°,  and  the  boiling-point  of  water 
373°,  these  numbers  have  the  same  ratio  to  each  other 
as  the  volumes  of  air  or  of  any  other  gas  at  these  tem- 
peratures. 

P.  Why  is  that? 

M.  You  only  need  to  look  at  your  diagram. 

P.  Oh,  I  understand.  The  diagram  has  been  made 
from  these  numbers. 

M.  So  the  volumes  of  gases  are  proportional  to  their 
absolute  temperatures. 

P.  That's  very  neat.  I  didn't  think  I  could  have 
learned  so  much  from  a  simple  drawing. 

M.  Do  you  see  why?  It  is  because  in  a  diagram 
everything  is  before  your  eyes,  whereas  in  words  or 
calculations  we  can  only  discuss  single  points.  You 
must  always  try  to  represent  general  relations,  or  laws 
of  nature  by  means  of  diagrams. 

P.  I'd  do  it  if  I  oilly  knew  how. 

M,  We  shall  have  other  examples. 


« 1 6  CONyERSA TIONS  ON  CHEMISTR Y. 

P.  Please  let  me  put  one  question  before  you  stop. 
You  have  been  speaking  again  as  if  temperature  was 
the  only  reason  why  the  volume  of  air  alters,  but  I  know 
that  pressure  changes  it  too.  What  happens  when  both 
change  at  the  same  time? 

M.  That  is  a  very  good  question.  You  want  to  know 
how  to  calculate  the  volume  of  a  gas  when  both  its  tem- 
perature and  pressure  change? 

P.  Yes. 

M.  Then  calculate  first  the  change  that  would  take 
place  if  only  the  pressure  were  altered  without  change  of 
temperature,  and  then  the  change  which  would  be  pro- 
duced in  the  resulting  volume  by  change  of  temperature 
at  constant  pressure. 

P.  Why  must  I  first  calculate  the  change  of  pressure? 

M.  You  might  just  as  well  calculate  the  change  of 
temperature  first. 

P.  Should  I  get  the  same  result? 

M.  Of  course.  The  volume  of  the  gas  depends  only 
on  the  temperature  and  the  pressure,  and  it  is  all  one 
in  which  order  they  are  taken. 

P.  That  looks  right,  but  I  don't  feel  quite  sure  about 
it. 

M.  Let  us  take  an  example.  Suppose  we  have  350  ccs. 
of  air  at  18°,  and  that  74.8  cm.  is  the  height  of  the  barom- 
eter, and  that  we  wish  to  know  what  the  volume  will  be 
at  0°  and  76.0  cms.  That  is  the  standard  temperature 
and  pressure  for  measuring  gases;  they  are  called  normal 
temperature  and  pressure.  Let  us  suppose  first  that 
according  to  Boyle's  law  the  volume  varies  inversely 
as  the  pressure.  Let  us  call  the  unknown  volume  at 
76  cms.  jy  then  ;y  1350  =74.8: 76.0. 

P.  Then  ^'-344. 


THE  EXPANSION  OF  AIR  BY  HEAT.  217 

M.  Furthermore,  the  volume  at  18°  is  to  the  volume 
at  0°  as  273+18  =  291  to  273.  If  you  call  the  volume 
at  0°  X,  you  have  the  proportion — 

P.  :x;  1344  =  273 1291,  therefore  :r=323. 

M.  Quite  right.  Now  you  can  reduce  the  volume 
350  CCS.  first  to  0°,  and  then  to  76  cms.  pressure,  and 
convince  yourself  that  you  get  the  same  answer. 

P.  We  have  been  talking  so  long  about  air  that  I 
have  almost  forgotten  that  these  are  supposed  to  be 
lessons  in  chemistry. 

M,  What  you  have  learnt  holds  for  all  gases.  If  you 
have  two  equal  volumes  of  any  two  gases  at  equal  tem- 
perature and  pressure,  their  volumes  always  remain 
equal,  however  you  alter  simultaneously  the  temperature 
and  pressure  of  the  gases. 

P.  It  is  quite  different  from  liquids,  for  water  expands 
differently  from  mercury. 

M.  Yes,  gases  show  no  peculiarities;  they  all  behave 
in  the  same  way.  You  will  see  later  that  the  behaviour 
of  gases  is  identical  in  many  respects  whatever  their 
chemical  differences. 

P.  Are  all  gases  colourless? 

M.  No,  I  have  told  you  already  that  chlorine  is  greenish, 
and  iodine  gas  violet.  But  I  must  tell  you  that  the 
conformity  is  confined  to  the  gaseous  state.  As  soon 
as  a  gas  condenses  to  a  liquid  the  differences  appear 
again,  for  one  gas  is  more  easily  condensed  than  another. 
The  same  is  the  case  when  gases  dissolve  in  water  or 
other  liquids.  But  as  long  as  the  substances  are  in 
the  state  of  gas,  the  uniformity  of  their  external  prop- 
erties is  maintained. 

P.  I'm  glad  you  have  answered  my  question.  For 
now  I  have  learnt  without  knowing  it  that  what  you 


2 1 8  CON  VERS  A  TIONS  ON  CHE  MIS  TR  Y. 

have  taught  me  about  air  appHes  to  all  other  gases.    Do 
vapours,  Hke  steam,  obey  the  same  laws? 
M.  Certainly  there  is  no  difference. 


26.  THE  WATER  IN  THE  AIR. 

M.  So  far  you  have  learnt  about  two  of  the  constitu- 
ents of  air,  oxygen  and  nitrogen,  but  these  are  not  all; 
there  are  others,  among  which  water  in  the  state  of  vapour 
is  important. 

P.  Yes,  I  wanted  to  ask  you  about  that.  The  pressure 
of  the  air  is  one  atmosphere,  and  at  that  pressure 
water  boils  at  ioo°.  How  does  it  happen  that  water- 
vapour  can  be  present  in  air  when  it  is  much  colder 
than  ioo°?  Why  doesn't  all  the  water- vapour  condense 
to  liquid  water? 

M.  I  am  very  glad  you  are  taking  such  pains  to  learn, 
for  I  have  given  you  a  hint  how  to  answer  that  ques- 
tion. It  happens  that  when  water  evaporates  only  the 
pressure  of  its  own  vapour  counts,  and  not  the  pressure 
of  other  gases  and  vapours  which  may  be  present  at  the 
same  time. 

P.  Please  explain  that  to  me. 

M.  Remember  what  I  told  you  before  about  the 
behaviour  of  water  in  a  vacuum  (page  176);  it  will 
evaporate  until  the  space  is  filled  with  vapour  of  a 
definite  density.  Now  if  any  other  gas  is  present  in 
the  same  volume,  for  example,  air  or  hydrogen,  the 
water-vapour  will  do  exactly  the  same  thing — it  will 
be  formed  until  it  has  filled  the  space.  Its  pressure 
will  be  added  to  the  pressures  which  the  other  gases 
exert,  and   the   final   pressure  will    be  the  sum  of  all. 


THE  IV^TER  IN   THE  AIR.  219 

Only  the  evaporation  is  somewhat  slower,  because  the 
vapour  requires  time  to  spread  or  diffuse  throughout 
the  other  gases. 

P.  I  think  I  understand,  but  I  should  like  to  see  it. 

M.  First  of  all,  you  can  easily  convince  yourself  that 
ordinary  air  really  contains  water  in  the  state  of  vapour. 
You  know  that  this  water  deposits  upon  cold  objects  in 
the  form  of  dew,  and  that  jain  falls  because  the  water- 
vapour  of  the  air  changes  into  liquid  water  when  it  is 
cooled. 

P.  So  water- vapour  can  be  removed  from  air  by  cool- 
ing it? 

M.  Quite  easily.  I  close  the  neck  of  a  small  flask 
with  a  cork,  through  which  pass  an  entry  and  exit  tube 
(Fig.  43),  and  make  a  freezing- mixture  with  powdered  ice 
and  salt,  in  the  proportion  of  3  to  i,  and  surround  the 
flask  with  it.  Then  I  only  need  to  draw  air  from  the 
room  through  the  flask  for  some  time  in  order  to  collect 
a  considerable  quantity  of  water  in  the  form  of  ice  in 
the  flask.  If  I  let  the  flask  warm  up,  of  course  I  get 
water. 

P.  But  how  can  I  make  a  stream  of  air  pass  through 
the  flask?     It  is  tiresome  to  suck  so  long. 

M.  We  will  use  our  gas-holder  for  it  (Fig.  28,  page  149). 
If  we  place  the  empty  bottle  on  the  floor,  and  connect 
the  other  with  it  by  means  of  an  india-rubber  tube, 
when  the  water  runs  into  the  lower  bottle  it  sucks  air 
into  the  upper  one.  We  can  regulate  the  rate  with  the 
clip.  And  if  we  want  to  suck  more  air  through,  we 
have  only  to  reverse  the  bottles,  and  connect  the  upper 
one  with  the  under  one  by  the  tube. 

P.  That's  capital.  It  didn't  occur  to  me  that  you 
could  suck  with  it  as  well  as  blow. 


220 


CONVERSATIONS  ON  CHEMISTRY. 


M.  Now  the  experiment  has  been  going  long  enough; 
you  see  a  quantity  of  frost  has  condensed  in  the  flask. 

P.  Is  cooling  the  air  the  only  way  to  take  water  out 
of  it? 


Fig.  43- 

M.  No,  it  can  be  done  in  other  ways.  There  are 
many  substances  which  combine  so  easily  with  water 
that  we  only  need  to  lead  moist  air  over  them  to  remove 
the  moisture.  Caustic  soda,  which  you  have  already 
seen,  is  such  a  substance  (page  70);  another  is  a  salt 
named  chloride  of  calcium,  which  is  made  in  large  quan- 
tities as  a  by-product  in  chemical  works.  When  it  is 
dried  or  fused  it  takes  water  so  quickly  from  the  air 
that  a  piece  left  in  an  open  dish  changes  in  half  an  hour 
into  a  liquid  drop.  Air  and  other  gases  can  be  very 
easily  dried  by  its  help. 

P.  How? 


THE  lVy4TER  IN   THE  AIR. 


221 


M.  The  salt  is  placed  in  a  specially  shaped  tube  (Fig. 
44)  and  the  gases  that  we  wish  to  dry  are  led  through  it. 
If  you  cannot  blow  such  tubes  for  yourself,  you  need 
only  close  a  wide  tube  at  both  ends,  with  perforated 
corks  having  narrow  tubes  through  them;    only  don't 


Fig.  44. 

forget  to  close  the  end  of  the  tube  with  cotton-wool  in 
order  that  particles  of  dust  from  the  salt  may  not  be 
carried  along  by  the  stream  of  air.  If  you  weigh  such  a 
tube  exactly,  and  then  pass  a  measured  quantity  of 
air  through  it  and  weigh  it  again,  you  can  find  out  how 
much  water  was  in  the  air. 

P.  I  shall  try  that. 

M.  You  will  not  find  much  unless  you  pass  a  couple  of 
dozen  litres  through  it. 

P.  How  much  water  is  there  in  the  air  ? 

M.  It  varies  greatly.  It  depends  on  the  temperature 
as  well  as  on  the  source  of  the  air.  Do  you  remember 
what  I  have  just  told  you  about  the  evaporation  of  water 
in  the  air  (see  page  218). 

P.  Yes,  exactly  as  much  evaporates  as  if  there  were 
no  air  present  in  the  space. 

M,  Quite  right.  You  know  then  that  the  pressure 
of  the  vapour,  and  consequently  the  amount  in  a  given 
volume,  will  be  greater  the  higher  the  temperature.     Here 


2  22  CONFERS  A  TIONS  ON  CHE  MIS  TR  Y, 

is  a  table  which  shows  you  how  many  grams  of  water- 
vapour  are  present  in  a  Htre  of  air  when  it  stands  in 
contact  with  liquid  water,  or,  as  we  say,  when  it  is  satu- 
rated with  water-vapour.  A  litre  of  saturated  air  con- 
tains 

At    o° o .0049-gram  water-vapour 

'     5° 0.0068    " 

*  10° 0.0094    "  *' 

*  15° 0.0127    "  " 

'  20° 0.0171     " 

*  25° 0.0228    "  " 

P.  The  word  saturated  is  the  same  that  you  use  for 
solutions. 

M.  It  is  really  the  same  thing,  for  it  means  that  the 
air  cannot  take  up  any  more  water-vapour. 

P.  But  it  can  take  up  less? 

M.  Certainly,    that   is   also    the   case   with   solutions. 

Ordinary  air  like  what  is  in  the  room  is  almost  always 

unsaturated;    it  is   only  saturated  when  rain  or  mist  is 

present.     The  proportion  between  the  amount  of  water 

in  the   air  and  the  quantity  required   for  saturation   is 

called   the  hygrometric  state  of  the  air.     When  air,   for 

example,  at  20°  contains  0.0140  gram  of  water  per  Htre 

•  .         .  t        0.0140 

its  moisture  is  equal  to   =  0.82,  or  82    per  cent, 

^  0.0171  '  ^  ' 

because,  according  to  the  table,  it  could  contain  0.0171 

gram.     Air   generally   contains    about    70    per   cent   of 

moisture;    if  it  contains  50  per  cent  we  call  it  dry,  and 

we  call  90  per  cent  damp. 

P.  I  understand  that. 

M.  Now  look  at  the  table  again;  if  the  temperature 
rises  10°  the  amount  of  water  is  nearly  doubled.  A 
sample  of  air  which  is  only  half  saturated  at  20°  is  almost 


THE   IVATER  IN   THE  AIR.  223 

completely  saturated  at  10°,  and  ordinary  air  with  70 
per  cent  of  moisture,  if  it  is  cooled  10°,  will  part  with  a 
good  lot  of  its  water  in  the  Hquid  state.  That  is  the 
reason  of  rain. 

P.  Numbers  make  such  a  lot  of  things  clearer  than 
words.     But  why  should  rain  come  and  not  mist? 

M.  That  depends  upon  how  much  water  has  to  be 
separated.  If  there  is  only  a  little,  the  very  small  drop- 
lets which  are  formed  do  not  unite  to  large  drops  and  the 
result  is  fog;  in  other  cases  rain  occurs,  but  fog  and 
mist  always  precede  rain;  we  call  the  fog  which  occurs 
in  the  upper  air  a  cloud. 

P.  How  do  you  know  that  clouds  are  only  mist? 

M.  The  tops  of  mountains  are  often  hidden  by  clouds, 
and  when  you  cKmb  them  you  find  the  clouds  are  nothing 
but  mist. 

P.  Please  tell  me  how  it  happens  that  air  is  not  com- 
pletely saturated  with  water-vapour.  It  is  always  touch- 
ing water,  either  the  sea  or  ponds  on  the  land. 

M.  That  depends  on  its  motion.  Suppose  you  had 
the  air  saturated  in  one  place,  if  it  moves  to  a  warmer 
place  it  will  become  unsaturated  as  you  can  see  from 
the  table,  or  if  it  moves  to  a  colder  place  it  loses  a  portion 
of  its  water  in  the  form  of  rain.  And  when  it  recovers 
its  original  temperature,  it  is  unsaturated  again.  So 
whatever  happens,  when  it  changes  it  can  only  change 
in  one  direction,  in  becoming  less  saturated. 

P.  That  is  far  simpler  than  I  thought  it  was. 


2  24  CONVERSATIONS  ON  CHEMISTRY. 


27.  CARBON. 

M.  The  element  carbon  is  as  widely  distributed  and 
as  important  as  oxygen,  hydrogen,  and  nitrogen.  You 
already  know  that  ordinary  charcoal  is  one  form  of  that 
element. 

P.  I  thought  you  would  be  speaking  of  carbon  to-day, 
and  so  I  looked  at  a  piece  of  charcoal.  I  noticed  one 
thing;  you  can  see  the  rings  in  the  charcoal  just  as  in 
the  wood. 

M.  Yes,  you  can  see  the  rings  which  show  the  number 
of  years  of  growth,  and  besides  that  you  can  see  under  a 
microscope  the  single  cells  of  which  the  wood  consisted. 

P.  But  surely  wood  doesn't  consist  only  of  carbon? 

M.  No;  it  is  a  compound  of  carbon,  hydrogen,  and 
oxygen.  In  charcoal-burning  as  it  is  called  the  wood  is 
slowly  heated,  and  carbon  alone  remains,  for  the  other 
two  elements  are  expelled.  But  as  carbon  melts  only 
at  a  very  high  temperature,  which  is  not  nearly  reached 
in  charcoal-burning,  the  remaining  charcoal  retains  the 
form  of  the  cells  of  which  the  wood  consisted.  More- 
over, wood  charcoal  is  not  pure  carbon.  You  see  that 
when  it  burns,  for  ashes  always  remain,  while  pure 
carbon  leaves  no  residue  on  burning. 

P.  Is  there  such  a  thing  as  pure  carbon? 

M.  Certainly ;  ignited  lampblack  is  nearly  pure  carbon. 
You  know  that  lampblack  is  a  very  fine  black  powder. 

P.  You  said  before  that  almost  all  pure  substances 
form  crystals,  but  lampblack  doesn't  look  crystaUine. 

M.  Nor  is  it.  Such  substances  are  called  amorphous, 
which  means  without  shape.  Lampblack  is  amorphous 
carbon;  so  is  wood  charcoal,  only  it  is  impure. 


CARBON.  225 

P.  Is  coal  carbon,  too  ? 

M.  No,  ordinary  coal  and  its  varieties,  anthracite, 
brown  coal,  and  peat,  are. all  chemical  compounds  which 
contain  a  large  percentage  of  carbon;  anthracite  con- 
tains most;  peat,  least.  They  all  owe  their  origin  to 
plants.  Indeed,  in  coal  the  remains  of  plants  are  not 
infrequent;  in  brown  coal  they  can  be  seen  even  more 
distinctly,  and  peat  sometimes  consists  almost  entirely 
of  stems  and  roots.  These  materials,  owing  to  their  being 
buried  a  long  time  in  the  earth,  have  undergone  almost 
the  same  changes,  as  wood  undergoes  on  being  carbonized 
by  heat,  only  the  change  is  a  much  slower  one. 

P.  Now  I  begin  to  see  why  you  told  me  that  carbon 
was  such  an  important  element.  All  fuel  consists  of  car- 
bon. 

M.  Quite  right.  But  fuel  is  used  not  merely  for  heat- 
ing, but  for  all  sorts  of  other  purposes.  All  machines 
except  those  driven  by  running  water,  like  water-mills, 
are  driven  by  means  of  carbon;  moreover,  in  chemical 
works  and  in  works  in  which  iron  and  other  metals 
are  smelted,  the  processes  are  all  carried  out  by  help 
of  carbon;  indeed,  the  progress  of  our  civilization  may 
be  said  to  depend  on  carbon. 

P.  Why  is  that?  I  mean  why  is  carbon  required  for 
all  such  purposes? 

if .  By  the  burning  of  carbon  a  large  amount  of  work 
is  made  available,  which  generally  appears  in  the  form  of 
heat.  By  help  of  such  work  machines  are  set  in  motion. 
Chemical  processes  are  carried  on  which  would  not  go 
of  their  own  accord;  in  short,  carbon  places  at  our  dis- 
posal quantities  of  energy  which  we  use  for  all  sorts  of 
work. 

P.  Why,  you  said  the  same  about  oxygen. 


226  CONVERSATIONS  ON  CHEMISTRY. 

M.  Energy  is  only  liberated  when  carbon  and  oxygen 
combine  with  each  other,  that  is,  when  carbon  burns. 
You  see  the  carbon  is  as  necessary  as  the  oxygen. 

P.  And  because  oxygen  is  a  gas,  it  is  everywhere 
round  us,  but  carbon  must  be  bought  because  it  is  a 
solid. 

M.  Well  done!  that  is  a  good  remark.  What  you 
say  is  quite  right.  But  you  see  this  gives  people  the 
power  of  putting  energy  where  it  is  wanted.  If  carbon 
were  all  round  us  in  the  form  of  gas,  as  air  is,  you  might 
perhaps  be  able  to  set  the  air  on  fire,  but  you  couldn't 
have  a  fire  in  a  fire-place. 

P.  They  would  explode. 

M.  Quite  right.  But  don't  let  us  speculate;  let  us 
think  of  what  actually  exists.  Carbon  is  the  most  im- 
portant source  of  energy  at  the  disposal  of  our  industries. 
Notice  this;  when  carbon  is  burned,  the  products  of  its 
combustion  are  sent  up  the  chimney  as  fast  as  possible, 
only  people  try  to  keep  in  the  heat  which  is  produced 
at  the  same  time  as  thoroughly  as  possible.  It  is  evi- 
dent then  that  coal  is  bought  not  on  account  of  the  carbon 
it  contains,  but  on  account  of  the  energy  which  it  can 
give  out. 

P.  It  never  struck  me  in  that  way  before.  But  I  see 
that  it  must  be  right. 

M.  You  know  that  a  steamer  or  a  locomotive  must 
carry  coal  with  it.  Each  can  go  only  as  far  as  the  coal 
will  permit.  If  the  coal  gives  out  the  engine  stops. 
And  so  there  are  islands  on  the  ocean,  and  stations  on 
the  coast,  where  ships  can  buy  more  energy  in  the  form 
of  coal. 

P.  But  if  I  were  to  row  a  boat  I  shouldn't  need  to  burn 
any  coal. 


CARBON.  227 

M.  You  know  the  answer  quite  well  yourself.  Think 
of  what  I  told  you  about  the  use  of  oxygen  in  supporting 
life. 

P.  Yes,  I  remember  that  food  does  the  same  as  coal. 
But  does  food  consist  of  carbon? 

M.  All  food  contains  carbon,  and  when  it  burns  it  gives 
up  energy,  just  as  when  it  passes  through  our  bodies. 
Food  consists  of  compounds  of  carbon,  hydrogen,  and 
oxygen;   sometimes  it  contains  nitrogen  too. 

F.  Yes,  I  remember.  Foods  that  give  a  disagreeable 
smell  on  burning  contain  nitiogen. 

M.  Yes.  As  food  is  also  used  for  the  building  up  of 
the  body,  all  substances  of  which  the  bodies  of  animals 
and  plants  consist  contain  carbon.  Such  substances 
are  called  organic  compounds^  because  animals  and  plants 
are  called  organisms. 

P.  Are  there  many  organic  compounds? 

M.  We  know  more  than  a  hundred  thousand,  and  new 
ones  are  being  discovered  every  day. 

P.  How  can  any  one  remember  them? 

M.  No  one  can,  of  course;  but  that  doesn't  matter; 
there  are  dictionaries  in  which  accurate  descriptions  of  all 
these  compounds  are  to  be  found. 

P.  Do  other  elements  form  as  many  compounds? 

M.  Not  by  a  long  way.  And  so  the  chemistry  of  car- 
bon compounds  is  treated  separately  from  that  of  the 
other  elements,  and  called  organic  chemistry,  while  the 
chemistry  of  other  substances  is  called  inorganic  chemistry. 

P.  Isn't  that  rather  an  arbitrary  division? 

M.  Not  so  much  as  it  looks.  Carbon  compounds 
have  certain  general  properties  which  bring  them  natu- 
rally into  one  group.  Moreover,  certain  simple  carbon 
compounds  are  treated  of  under  the  head  of  inorganic 


2  28  CONVERSATIONS  ON  CHEMISTRY. 

chemistry,  because  carbon  occurs  in  many  minerals  and 
rocks. 

P.  Yes,  as  coal  and  peat. 

M.  No,  in  other  chemical  compounds.  Marble  and 
chalk  contain  carbon.  But  we  shall  come  to  that  later; 
in  the  mean  time  let  us  consider  the  uncombined  element. 
We  must  now  consider  a  new  phenomenon  which  you 
ought  to  learn  about.  Do  you  know  that  the  diamond 
is  nothing  but  carbon  ? 

P.  Yes,  because  it  can  be  burned  when  it  is  heated 
very  hot. 

M.  That  is  not  a  sufficient  reason,  because  many  other 
substances  can  be  burned  at  a  high  temperature  although 
they  are  not  carbon.  Oxygen,  you  know,  unites  with 
most  other  elements. 

P.  Yes,  but  when  a  diamond  burns  I  have  read  that 
nothing  is  left. 

M.  That  is  a  better  test.  We  could  infer  that  the 
oxygen  compound  or  the  oxide  of  the  element  or  elements 
of  which  the  diamond  consists  is  volatile.  But  carbon 
is  not  the  only  element  which  leaves  no  residue  in  burning. 
Sulphur  and  hydrogen  give  volatile  oxides. 

P.  It  must  depend  then  upon  what  is  formed. 

M.  Quite  right,  now  you  are  getting  nearer  it.  When 
carbon  burns  a  gas  is  evolved  which  is  called  carbon 
dioxide;  we  have  talked  about  it  already  (page  68). 
It  can  be  easily  recognized  because  it  gives  a  white  pre- 
cipitate with  lime-water,  and  makes  clear  lime-water 
milky.  To  remind  you,  I  will  make  the  experiment 
somewhat  differently;  here  is  a  fragment  of  charcoal  in 
a  glass  tube.  Now  I  will  heat  the  place  wh^e  it  lies 
and  pass  air  over  it  from  the  gas-holder  (page  220). 
The  tube  is  bent  so  that  the  end  dips  under  lime-water 


CARBON. 


229 


in  the  glass.  Now  the  carbon  begins  to  glow,  and  you 
see  the  lime-water  is  turning  turbid. 

P.  If  I  were  to  heat  a  diamond  instead  of  charcoal, 
would  it  burn  and  turn  the  lime-water  milky? 

M.  Yes,  it  would,  only  you  couldn't  make  the  experi- 
ment in  an  ordinary  glass  tube,  because  the  diamond 
would  not  catch  fire  at  so  low  a  temperature;  the  tube 
would  melt.  Moreover,  you  would  require  to  use  pure 
oxygen,  for  the  diamond  would  then  burn  more  easily. 


Fig.  45. 


P.  Yes,  I  see  the  proof  that  the  diamond  is  really 
carbon. 

M.  Stop  a  little;  not  quite  so  quick.  You  would  only 
prove  that  the  diamond  contains  carbon,  but  not  that 
it  contains  nothing  but  carbon.  How  could  you  find  out 
that  there  are  no  other  elements  present? 

P.  I  don't  quite  understand  you. 

M.  Look  here.  I  shall  repeat  my  former  experiment 
with  a  piece  of  wood.  It  catches  fire,  too,  and  the  lime- 
water  again  turns  milky.  Yet  I  cannot  say  that  the 
wood  is  carbon,  but  only  that  it  contains  carbon;  for 
it  also  contains  hydrogen  and  oxygen. 

P.  Let  me  think.  Now  I  have  it;  water  must  be  pro- 
duced by  the  burning  of  the  hydrogen.     If  carbon  di- 


230  CONVERSATIONS  ON  CHEMISTRY, 

oxide  is  the  only  product,  we  know  that  the  substance 
contains  only  carbon. 

M.  That  is  more  nearly  correct,  but  not  quite.  The 
diamond  might  be  a  compound  of  carbon  with  less 
oxygen  than  is  contained  in  carbon  dioxide.  Such  a 
compoujid  on  burning  would  give  only  carbon  dioxide, 
and  yet  it  would  not  consist  of  carbon  alone. 

P.  Is  there  such  a  compound? 

M.  Certainly,  but  it  is  not  a  soHd  hard  substance  like 
a  diamond,  but  a  gas. 

P.  Then  there's  no  risk  of  mistaking  it  for  a  diamond  ? 

M.  You  are  trying  to  evade  the  point.  That  is  unad- 
visable,  because  you  will  miss  an  opportunity  of  learning 
something. 

P.  I  must  say  I  don't  understand  the  question  of  the 
diamond. 

M.  When  carbon  bums,  3  parts  by  weight  of  carbon 
always  give  11  parts  of  carbon  dioxide,  for  it  com- 
bines with  8  parts  of  oxygen;  now  with  the  diamond 
that  is  the  proportion.  If  the  diamond  contained  some 
element  besides  carbon  it  would  give  less  carbon  dioxide, 
and  indeed  the  amount  would  be  proportional  to  the 
amount  of  carbon  it  contains. 

P.  Then  wood  must  give  much  less  carbon  dioxide  than 
charcoal. 

M.  So  it  does;  3  parts  by  weight  of  wood  give  at  most 
4j  parts  of  carbon  dioxide. 

P.  And  no  single  substance  is  known  which  gives 
more? 

M.  Not  one.  But  there  is  another  substance  which 
gives  exactly  the  same  amount,  namely,  graphite,  of 
which  ordinary  lead  pencils  are  made. 

P.  Then  it  must  be  carbon? 


CARBON.  231 

M.  Yes,  we  must  therefore  say  that  carbon  appears 
in  three  different  forms:   charcoal,  diamond,  graphite. 

P.  But  I  don't  understand  that.  How  can  one  and 
the  same  substance  exist  in  three  different  forms,  and 
why  do  people  not  make  diamonds  from  charcoal  if  it 
contains  only  carbon? 

M.  That  is  a  very  good  question  and  I  will  answer 
you  as  well  as  I  can.  You  already  know  that  one  and 
the  same  substance,  for  example,  water,  can  exist  in 
different  forms.  Water,  indeed,  exists  in  three,  namely, 
ice,  water,  and  steam. 

P.  Yes,  there  are  three  forms,  but  carbon,  diamond, 
and  graphite  are  all  solid  bodies.  But  if  all  three  could 
be  changed  into  each  other  by  heating  or  cooling  then  I 
would  beheve  it.  But  they  can  all  exist  together  at  the 
same  temperature. 

M.  You  are  quite  right.  Nevertheless  charcoal  can 
really  be  changed  into  graphite;  it  takes  place  at  a  very 
high  temperature. 

P.  Can  you  show  me  that? 

M.  It  is  not  so  difficult.  The  carbon  rods  used  for 
electric  lamps  consist  of  ordinary  carbon.  The  next 
time  the  workman  puts  in  new  carbons  ask  him  for  one 
of  the  old  ends.  You  will  see  that  the  points  have 
become  grey  and  smooth  and  shine  something  Kke  a 
metal;  they  have  changed  into  graphite.  The  carbon 
threads  of  an  electric  glow  lamp  have  undergone  the 
same  change  owing  to  the  high  temperature  at  which 
they  have  been  kept.  Indeed,  they  are  made  of  cotton 
carbonized,  and  after  they  have  served  their  purpose  they 
become  grey  and  shining  like  graphite. 

P.  Next  time  a  lamp  burns  out  I  will  ask  for  it  and 
break  it  open. 


232  CONVERSATIONS  ON  CHEMISTRY. 

M.  Take  care  that  you  don't  lose  the  very  fine  thread. 

P.  Can  diamond  also  be  changed  into  graphite? 

M.  Yes,  in  exactly  the  same  way,  by  heating  it  strongly. 

P.  And  can  the  change  be  reversed? 

M.  Graphite  can  only  be  changed  into  ordinary  carbon 
by  a  roundabout  process.  It  has  to  be  made  into  com- 
pounds, and  the  carbon  separated  again  from  them. 

P.  I  don't  quite  understand  that. 

M.  I  will  not  try  to  explain  it  to  you  now,  since  sub- 
stances must  be  used  which  you  do  not  know  about. 
You  must  be  contented  in  the  mean  time  in  believing 
that  it  is  possible. 

P.  But  how  is  it  with  diamonds?  Can  they  be  made 
from  charcoal  or  graphite? 

M.  That  is  also  possible. 

P.  Then  why  aren't  diamonds  cheap? 

M.  There  is  no  danger  of  that,  because  only  minute 
diamonds  can  be  made,  and  in  very  small  quantity. 

P.  But  why  is  that?     Charcoal  is  cheap  enough. 

M.  Yes,  that  brings  us  again  to  our  general  question. 
I  compared  the  three  states  of  carbon  with  the  forms 
of  matter.  Now  carbon  is  also  able  to  become  liquid 
and  gaseous,  so  that  the  ordinary  forms  of  matter  are 
known  in  its  case. 

P.  Liquid  and  gaseous  carbon! 

M.  Yes,  but  a  very  high  temperature  is  necessary — 
more  than  3000°;  however,  it  can  be  reached  by  the 
electric  current.  You  see  then  that  carbon  can  exist 
in  the  gaseous,  Hquid,  and  soUd  states.  Carbon  is  known 
not  only  in  three,  but  in  five,  different  states. 

P.  So  that's  how  it  is.  Yes,  I  see.  Just  as  water 
can  be  changed  to  steam  by  heating  it,  so  charcoal  changes 
to  graphite  by   heating;    no,   I   don't   see   yet.     When 


C/iRBON.  233 

graphite  cools,  it  doesn't  go  back  to  charcoal,  but  remains 
as  it  is. 

M.  Yes,  that  is  the  most  difficult  side  of  the  question, 
but  I  think  I  can  explain  it  to  you.  You  know  that 
water  changes  into  ice  at  0°.  Do  you  remember  what  I 
told  you  about  super-cooHng  (page  164). 

P.  Yes,  that  water  can  be  cooled  below  0°,  and  remains 
liquid  if  no  ice  is  present. 

M.  Right.  I  have  here  a  sealed  glass  tube  containing 
water  into  which  no  ice  can  enter.  I  now  place  the  tube 
in  a  mixture  of  ice  and  water  which  of  course  is  at  0°, 
and  I  can  leave  it  as  long  as  I  like;  ice  will  never  be 
formed. 

P.  That  won't  do,  you  must  cool  it  below  0°. 

M.  Quite  right.  Now  if  I  add  some  salt  the  tem- 
perature will  fall  below  0°.  Half  a  teaspoonful  will  do; 
the  thermometer  shows  —4°,  and  there  is  no  sign  of  the 
water  freezing. 

P.  But  if  you  were  to  leave  it  for  a  long  time  ? 

M.  Nothing  would  happen.  If  I  were  to  add  more 
salt  so  as  to  lower  the  temperature  to  —10°,  and  were 
then  to  shake  it  violently  ice  would  be  formed. 

P.  Yes,  I  see  that. 

M.  Now  with  a  diamond,  you  must  think  in  the 
same  way.  The  conditions  of  our  experiments  are 
such  that  the  diamond  cannot  be  formed.  In  order 
to  produce  it  a  very  great  pressure  and  a  very  high 
temperature  are  necessary.  These  conditions  are  diffi- 
cult to  obtain,  and  hence  it  is  very  difficult  to  make 
diamonds. 

P.  Yes,  I  can  understand  that.  But  why  is  carbon 
the  only  substance  which  behaves  like  that? 

M.  Carbon  is  not  the  only  substance.    You  will  soon 


234  CONVERSATIONS  ON  CHEMISTRY. 

become  acquainted  with  other  substances  which  also  exist 
in  several  solid  forms. 

P.  Are  such  different  forms  only  known  in  the  case 
of  solid  substances? 

M,  For  the  most  part.  Such  substances  are  called 
allotropic.  Charcoal,  diamond,  and  graphite  are  allo- 
tropic  forms  of  carbon. 

P.  Now  I  think  I  understand  it  a  little.  But  I  want 
to  ask  one  question:  What  do  these  differences  depend 
on — what  are  they  connected  with? 

M.  With  the  differences  in  the  amount  of  work  or 
energy  in  the  substances.  Just  as  work  is  required  to 
change  ice  into  water,  or  water  into  steam,  so  energy 
is  required  to  change  charcoal  into  diamond;  and  no 
second  substance  takes  part  in  this  change  in  either 
case. 

P,  Could  we  not  regard  energy  as  a  kind  of  chemical 
element  which  combines  with  a  substance,  and  gives  it 
different  properties? 

M.  That  is  one  way  of  looking  at  it,  but  energy  pos- 
sesses no  weight,  and  therefore  during  such  allotropic 
changes  there  is  no  change  of  weight. 

P.  Now  I  think  I  understand  everything. 


28.  CARBON  MONOXIDE. 

M.  You  have  seen,  several  times,  what  happens  when 
carbon  is  burned. 

P.  Yes,  a  gas  is  formed  named  carbon  dioxide,  which 
consists  of  carbon  and  oxygen.     Why  is  it  called  dioxide  ? 

M.  Because  there  is  another  compound  of  the  two 
elements  which  is  called  carbon  monoxide.     The  dioxide 


CARBON  MONOXIDE,  235 

contains  twice  as  much  oxygen  as  the  monoxide.  The 
syllables  mon  and  di  are  Greek  prefixes  meaning  one 
and  two. 

P.  What  is  carbon  monoxide  like? 

M.  It  is  a  colourless  gas,  but  differs  from  the  dioxide 
by  being  combustible.    Moreover,  it  is  very  poisonous. 

P.  Can  I  see  it? 

M.  Yes,  if  you  can  speak  of  seeing  a  gas;  it  is  colour- 
less so  that  you  cannot  distinguish  it  from  air  in  appear- 
ance ;  its  density  and  its  other  physical  properties  resemble 
those  of  nitrogen.     But  you  have  often  seen  it  burning. 

P.  When  and  where? 

M.  You  have  often  seen  coal  burning  in  the  fire.  At 
first  you  know  it  gives  out  a  bright  flame  which  comes 
from  the  burning  of  compounds  of  carbon  and  hydrogen 
which  resemble  coal-gas,  and  which  give  the  flame  its 
brightness. 

P.  Yes,  of  course. 

M.  After  all  the  coal  glows  red-hot  the  flame  changes 
its  appearance  and  becomes  pale  blue  in  colour. 

P.  Yes,  I  have  noticed  that.  It  looks  like  the  flame 
of  a  spirit-lamp. 

M.  Yes,  that  is  the  flame  of  carbon  monoxide  burning. 
At  first  the  oxygen  of  the  air  combines  with  the  carbon, 
of  the  coal,  to  form  dioxide;  but  the  dioxide  in  passing 
through  the  red-hot  coal  combines  with  the  carbon  and 
forms  carbon  monoxide;  then  the  carbon  monoxide 
burns  at  the  back  of  the  fire  to  carbon  dioxide  when  it 
comes  in  contact  with  more  oxygen. 

P.  I  must  look  more  carefully  at  that. 

M.  Do  so,  and  think  of  this.  Carbon  monoxide  is 
like  nitrogen  because  it  has  no  smell;  but,  as  I  told  you, 
it  is  very  poisonous.     If  it  escapes  into  the  room  much 


^$6  CONVERSATIONS  ON  CHEMISTRY. 

harm  may  be  done,  and  every  year  people  die  of  poisoning 
by  carbon  monoxide. 

P.  How  does  that  happen? 

M.  It  seldom  happens  with  an  open  fire  unless  the 
damper  in  the  chimney  is  shut.  But  in  a  stove,  if  a 
sufficient  quantity  of  air  is  not  let  in  to  burn  the  carbon 
to  dioxide,  the  monoxide  is  formed,  which  may  escape 
into  the  room  and  poison  the  people  in  it. 

P.  But  surely  the  amount  of  carbon  monoxide  in  a 
room  must  be  very  small,  because  the  volume  of  the 
room  is  so  very  much  larger  than  that  of  the  stove,  and 
besides  air  is  always  entering  through  the  cracks  of  the 
door-  and  the  window-frames. 

M.  Quite  right,  but,  unfortunately,  carbon  monoxide 
is  absorbed  by  the  blood  even  when  very  little  is  con- 
tained in  the  air.  People  who  are  poisoned  by  carbon 
monoxide  show  no  signs  of  suffocation,  but  only  become 
dull  and  sleepy  and  get  headaches,  and  do  not  realize 
what  it  is  and  try  to  escape. 

P.  Can  anything  be  done  with  people  who  are  poisoned  ? 

M.  The  best  way  is  to  take  them  as  quickly  as  possible 
into  the  open  air,  and  make  them  draw  deep  breaths. 
If  they  are  far  gone  oxygen  may  be  given  if  it  is  at  hand, 
or  artificial  respiration  may  be  applied  in  the  same  way 
as  for  the  recovery  of  the  drowning,  by  moving  the  arms 
up  and  down  regularly.  Don't  forget  that  coal-gas 
generally  contains  a  good  deal  of  carbon  monoxide,  which 
makes  it  poisonous.  But  in  general  the  smell  is  suffi- 
cient warning,  although  it  is  due  to  other  constituents 
of  the  coal-gas. 

P.  Isn't  it  curious  that  a  compound  of  carbon  and 
oxygen  should  be  poisonous  when  neither  of  the  elements  is 
poisonous,  and  when  our  bodies  largely  consist  of  them  ? 


CARBON  DIOXIDE.  237 

M.  No,  it  is  only  another  example  of  the  fact  that  the 
properties  of  compounds  are  entirely  different  from 
those  of  their  elements.  I  remember  telling  you  before 
that  it  isn't  correct  to  speak  as  if  the  elements  were 
contained  in  their  compounds. 

P.  Yes,  I  remember,  too,  but  it  is  very  difficult  to 
change  one's  ordinary  way  of  speaking. 


29.  CARBON  DIOXIDE. 

M.  Do  you  remember  what  we  have  learned  about 
carbon  dioxide? 

P.  Yes,  it  is  formed  when  charcoal  bums,  or  when 
any  substances  containing  carbon  are  burnt.  It  can  be 
tested  for  with  lime-water. 

M.  You  have  remembered  that  very  well.  What  does 
the  lime-water  look  like  after  it  has  been  treated  with 
carbon  dioxide? 

P.  It  becomes  milky. 

M,  Yes.  In  the  chemical  language  we  say  that  a 
white  precipitate  is  formed. 

P.  What  is  precipitated? 

M.  If  you  let  it  stand  the  milkiness  would  settle  to 
the,  bottom  as  a  white  layer,  for  it  is  heavier  than  water. 
A  solid  substance  which  is  produced  in  a  liquid  by  a 
chemical  process  is  called  a  precipitate.  What  does 
carbon  dioxide  look  like? 

P.  A  colourless  gas. 

M.  Yes.  It  has  the  pecuhar  property  of  being  heavier 
than  air  and  so  it  behaves  in  a  manner  different  from 
hydrogen,  for  it  sinks  in  air  instead  of  rising  like  hydrogen. 

P.  I  should  like  to  see  that. 


238  CON  VERSA  TIONS  ON  CHE  MIS  TR  Y, 

M.  We  must  first  make  some  carbon  dioxide  for  that 
purpose.  I  shall  use  a  flask  exactly  Hke  the  one  I  used 
for  making  hydrogen  (Fig.  24,  page  135)  only  instead 
of  putting  zinc  in  the  flask  I  use  chalk  or  marble;  the 
funnel  contains  dilute  hydrochloric  acid.  You  see  that 
it  begins  to  froth  as  soon  as  I  let  hydrochloric  acid  into 
the  flask;    the  gas  which  is  evolved  is  carbon  dioxide. 

P.  What  does  the  hydrochloric  acid  do  to  the  chalk  ? 

M.  I  won't  explain  that  until  you  know  more;  but  you 
will  soon  learn.  We  shall  first  make  sure  that  the  gas 
which  is  evolved  is  really  carbon  dioxide.  I  am  passing 
it  into  an  empty  flask,  and  now  I  pour  in  some  lime-water 
and  shake  it. 

P.  Yes,  I  see,  that  is  the  white  precipitate. 

M.  This  experiment  shows  you  at  once  that  carbon 
dioxide  is  heavier  than  air,  for  it  has  stayed  in  the  flask. 
But  I  can  show  you  this  better  by  filling  two  test-tubes 
with  the  gas  just  as  we  did  with  hydrogen  (page  137) 
and  leaving  one  witH  its  mouth  upwards  and  the  other 
with  its  mouth  downwards.  This  time  it  is  the  one  with 
its  mouth  upwards  that  remains  full.  How  could  you 
find  that  out? 

P.  I  could  test  with  lime-water. 

M.  You  could  do  it  even  more  simply.  Carbon  di- 
oxide puts  out  a  burning  splinter.  Look,  I  thrust  a  burn- 
ing match  up  into  the  tube  with  its  mouth  downwards ;  it 
goes  on  burning.  But  it  goes  out  when  I  put  it  into 
the  tube  with  its  mouth  upwards. 

P.  Then  the  same  test  does  for  carbon  dioxide  as  for 
nitrogen. 

M.  Yes,  so  far  as  the  burning  splinter  is  concerned. 
But  they  behave  differently  with  lime-water,  for  nitrogen 
gives  no  precipitate  with  it.     It  is  not  uncommon  for  two 


CARBON  DIOXIDE.  239 

substances  to  behave  similarly  towards  one  test,  but  if 
they  differ  in  any  respect  they  must  be  different  sub- 
stances. There  are  many  other  differences  between 
these  gases.  Carbon  dioxide,  for  instance,  is  heavier 
than  nitrogen. 

P.  Why  did  the  splinter  go  out  in  carbon  dioxide? 
Doesn't  the  dioxide  contain  oxygen? 

M.  That  is  a  good  question.  You  know  the  splinter 
consists  largely  of  carbon ;  now  that  carbon  would  require 
to  displace  the  carbon  in  the  carbon  dioxide,  in  which 
it  is  already  combined  with  oxygen.  It  is  almost  as  if 
you  were  trying  to  raise  yourself  into  the  air. 

P.  Oh! 

M.  But  other  substances  can  take  away  oxygen  from 
carbon  dioxide.  You  have  seen  magnesium  ribbon  which 
burns  so  brightly.     I  fill  a  flask  with  carbon  dioxide — 

P.  Why  don't  you  collect  the  gas  over  water? 

M.  That  is  not  necessary ;  it  is  so  heavy  that  it  stays  at 
the  bottom  of  the  flask.  And  I  know  that  the  flask  is 
full  because  it  puts  out  a  burning  spail  when  I  hold  it 
to  the  mouth;  the  flask  is  full,  and  the  carbon  dioxide 
is  running  over. 

P.  That's  an  easy  way  of  doing  it!  Let  me  try;  yes, 
now  the  flask  is  full. 

M.  Now  I  fold  several  pieces  of  magnesium  ribbon 
together  (for  a  single  piece  goes  out  too  easily),  Hght  it, 
and  dip  it  into  the  carbon  dioxide. 

P.  It  hisses  and  sparkles! 

M.  You  see  that  it  burns  quite  differently  from  what 
it  did  in  the  air.  There  are  white  and  black  particles; 
the  white  particles  are  oxide  of  magnesium,  the  black 
particles  are  the  carbon  from  the  carbon  dioxide. 

P,  Oh,  mav  I  look  at  that? 


24©  CONVERSATIONS  ON  CHEMISTRY. 

M.  Wait  a  little.  I  have  poured  some  hydrochloric 
acid  on  it;  it  dissolves  the  magnesium  oxide,  and  leaves 
the  carbon. 

P.  Yes,  it  has  become  quite  black.  What  made  that 
frothing? 

M.  It  was  a  little  piece  of  metallic  magnesium,  which 
acts  like  zinc  upon  the  hydrochloric  acid,  and  evolves 
hydrogen.  Now  I  will  show  you  another  property  of 
carbon  dioxide.  I  fill  a  flask  with  the  gas  over  water,  let 
a  httle  more  water  enter,  close  the  mouth  with  my  thumb, 
and  shake.  You  see  my  thumb  sticks  to  the  mouth,  as 
if  it  were  sucked  in;  that  shows  that  the  pressure  in  the 
flask  has  decreased.  When  I  dip  the  neck  under  water 
and  remove  my  thumb,  a  good  deal  of  water  enters.  Now 
I  can  repeat  this  till  at  last  the  flask  is  almost  complete- 
ly filled  with  water.  What  does  this  experiment  show 
us? 

P.  That  carbon  dioxide  is  dissolved  in  the  water. 

M.  Yes,  it  is  pretty  soluble.  A  litre  of  water  at  the 
ordinary  temperature  absorbs  nearly  a  litre  of  carbon 
dioxide;  it  absorbs  a  little  more  when  it  is  cold,  and 
less  when  it  is  warm. 

P.  Is  that  not  the  way  soda-water  is  made?  I  think 
I  remember  your  telling  me  that. 

M.  Yes,  soda-water  is  a  solution  of  carbon  dioxide 
in  water. 

P.  But  doesn't  it  contain  soda? 

M.  It  used  to  contain  soda,  but  now  it  is  merely  a 
solution  of  carbonic  acid  in  water.  The  name  carbonic 
acid,  although  it  is  commonly  used,  should  not  be  applied 
to  the  gas,  but  only  to  its  solution  in  water.  Why  does 
soda-water  effervesce?  Do  you  remember  what  I  told 
you  about  that? 

P.  Yes,  you  told  me  that  the  bottles  are  filled  at  a  high 


CARBON  DIOXIDE.  241 

pressure  with  the  gas,  and  when  they  are  open  the  pres- 
sure decreases  and  the  gas  comes  out.  I  remember, 
too,  that  you  said  that  the  same  volume  of  gas  is  dissolved 
whatever  the  pressure  is. 

M.  Quite  right.  You  learned  that  at  any  given  tem- 
perature the  weights  of  a  gas  which  fill  a  given  volume 
are  proportional — 

P.  To  the  pressures? 

M.  Yes.  If  equal  volumes  are  always  dissolved  at  dif- 
ferent pressures,  what  will  the  weights  be  proportional  to  ? 

P.  To  the  pressures. 

M.  Quite  right.  So  at  different  pressures  different 
weights  of  gas  will  be  dissolved,  and  these  weights  are 
proportional  to  the  pressures.  Soda-water  has  generally 
a  pressure  of  four  atmospheres;  therefore  it  contains 
four  times  as  much  carbon  dioxide  as  it  can  retain  under 
a  pressure  of  one  atmosphere.  This  excess  escapes  on 
opening  the  bottle,  and  produces  the  frothing. 

P.  Some  other  liquids  foam;  for  instance,  beer.  Does 
that  depend  upon  carbon  dioxide,  too? 

M.  Yes,  but  the  gas  is  not  pumped  into  the  beer, 
but  is  formed  in  the  beer  from  malt,  and  remains  dis- 
solved in  the  liquid. 

P.  Then  where  does  it  come  from? 

M.  There  is  sugar  in  malt,  and  by  the  action  of  yeast 
this  is  decomposed  into  alcohol,  which  gives  the  beer  its 
intoxicating  properties,  and  into  carbon  dioxide,  some 
of  which  is  evolved.  In  beer- cellars  they  sometimes  use 
iron  bottles  filled  with  liquid  carbon  dioxide  for  driving 
the  beer  out  of  the  casks. 

P.  Liquid  carbon  dioxide? 

M.  Yes,  when  carbon  dioxide  is  compressed  with  a 
powerful  pump  it  turns  Hquid  like  water,  and  indeed  has 
almost  the  same  appearance. 


242  CONVERSATIONS  ON  CHEMISTRY. 

P.  It  must  be  a  very  strong  pump. 

M.  The  pressure  depends  on  the  temperature.  At  o°, 
35.4  atmospheres  are  required;  at  20°,  58.8,  but  at  —80° 
carbon  dioxide  Hquefies  at  i  atmosphere  pressure.  Liquid 
carbon  dioxide  boils  at  —80°.  It  behaves  exactly  like 
water,  for  its  vapour  has  a  higher  pressure  the  higher  the 
temperature.  Only  the  corresponding  temperatures  for 
carbon  dioxide  He  much  lower. 

P.  Should  we  call  carbon  dioxide  a  vapour? 

M.'  You  may  if  you  like. 

P.  Couldn't  you  bring  me  some  liquid  carbon  dioxide 
home  in  a  bottle  to  see  what  it  is  like? 

M.  That  would  be  impossible,  for  when  it  is  allowed 
to  escape,  out  of  the  steel  bottle,  it  becomes  solid  like 
snow. 

P.  How  is  that? 

M.  You  know  that  on  boiling  a  liquid,  heat  is  absorbed, 
all  liquids  behave  in  the  same  way  in  this  respect,  and 
carbon  dioxide  is  no  exception.  As  soon  as  liquid  carbon 
dioxide  is  exposed  to  air  which  has  only  one  atmos- 
phere pressure,  it  begins  to  boil  violently,  and  so  much 
heat  is  absorbed  by  the  portion  which  evaporates  that 
the  residue  freezes. 

P.  Then  it  should  be  possible  to  freeze  water  by 
boiling  it!     Surely  that  could  never  be  done! 

M.  It  is  not  difficult,  only  care  must  be  taken  that  the 
water  shall  boil  below  0°;  and  to  accomplish  that  the 
pressure  must  be  very  low.  As  a  matter  of  fact  water 
can  be  frozen  if  it  is  brought  into  a  space  as  free  from 
air  as  possible;  and  then  it  behaves  exactly  as  I  have 
told  you  that  carbon  dioxide  does.  Indeed,  there  are 
ice  machines  in  which  ice  can  be  made  in  summer  by 
this  process.     You  see  carbon  dioxide  resembles  water 


CARBON  DIOXIDE.  243 

in  existing  in  all  three  forms.  Liquid  carbon  dioxide 
has  become  a  valuable  article  of  commerce  for  aerating 
water  and  for  forcing  beer  out  of  casks,  and  if  you  look 
you  will  often  see  the  steel  flasks  filled  with  liquid  carbon 
dioxide  being  carted  about  on  the  streets. 

P.  Where  does  it  chiefly  come  from? 

M.  It  pours  out  of  the  earth.  In  many  places,  espe- 
cially where  there  are  or  have  been  volcanoes,  pure 
carbon  dioxide  issues  continuously  from  the  soil.  When 
it  comes  in  contact  with  subterraneous  springs,  the  water 
becomes  saturated  with  the  gas  and  escapes  as  carbonated 
or  sparkling  water. 

P.  Why  does  it  taste  sour? 

M.  A  solution  of  carbon  dioxide  has  an  acid  taste. 

P.  Is  that  why  it  is  called  carbonic  acid? 

M.  That  has  to  do  with  it.  Sometimes  carbon  dioxide 
issues  from  the  earth  as  a  gas,  and  can  be  compressed 
into  steel  flasks  with  the  help  of  powerful  pumps.  There 
are  such  carbon  dioxide  wells  at  Naples,  in  the  neighbour- 
hood of  Vesuvius.  There  is  a  cave,  the  floor  of  which 
is  somewhat  depressed,  into  which  the  gas  pours  until 
it  fills  the  depression  nearly  two  feet,  and  the  gas  flows 
out  over  the  floor  just  as  if  it  were  water.  People  can 
walk  about  in  this  grotto  without  danger,  because  their 
heads  are  above  the  level  of  the  carbon  dioxide,  but 
dogs  are  suffocated,  as  they  are  at  a  lower  level.  That 
is  the  well-known  ''Grotto  del  Cane,"  or  *'Cave  of  the 
Dog." 

P.  Do  they  really  let  dogs  suffocate  in  it? 

M.  No,  they  bring  them  out  before  they  are  dead, 
and  revive  them  by  splashing  them  with  water. 

P.  How  cruel!  Why  are  dogs  suffocated  by  carbon 
dioxide  ? 


244  CONVERSATIONS  ON  CHEMISTRY. 

M.  For  the  same  reason  that  they  die  in  nitrogen; 
because  they  can  get  no  oxygen  to  breathe.  Carbon 
dioxide  isn't  really  a  poison  any  more  than  nitrogen, 
because  it  is  always  present  in  our  lungs. 

P.  How  does  it  get  there? 

M.  Out  of  the  blood.  I  have  already  told  you  that 
the  food  which  we  eat  contains  carbon  and  that  it  is 
burnt  in  our  tissues  by  means  of  the  oxygen  which  the 
blood  leads  to  it.  It  burns  to  carbon  dioxide  just  as  in 
ordinar}^  combustion;  the  gas  is  absorbed  by  the  blood, 
and  we  breathe  it  out  from  our  lungs  along  with  nitrogen. 

P.  So  carbon  dioxide  is  present  in  the  air  which  I 
breathe  out? 

M.  Certainly;  blow  some  air  through  a  glass  tube 
into  lime-water. 

P.  So  it  is,  the  lime-water  becomes  milky,  and  there  is 
a  white  precipitate.    How  much  I  have  to  think  about ! 


30.  THE  SUN. 

P.  I  have  been  puzzling  my  head  ever  since  the  last 
lesson.  I  know  now  that  carbon  dioxide  is  produced 
by  combustion,  by  breathing,  and  by  decay,  and  that 
in  some  places  it  streams  out  of  the  earth.  It  must  all 
collect  in  the  air,  and  accumulate.  Isn't  the  air  full  of 
carbon  dioxide? 

M,  There  is  always  some  in  the  air,  but  not  very 
much;  only  about  4  parts  in  10,000.  More  is  present  in 
close  rooms  when  much  carbon  dioxide  has  been  produced 
by  breathing  or  by  the  burning  of  gas.  You  can  easily 
recognize  it  by  exposing  some  lime-water  to  the  air  in  the 
room,  for  it  will  become  covered  over  with  a  white  scum. 


THE  SUN. 


^45 


P.  Covered  over?  Oh,  I  see;  because  the  carbon 
dioxide  can  only  act  on  the  surface  of  the  water.  But 
what  becomes  of  all  the  carbon  dioxide  that  is  poured 
into  the  air?  Perhaps  the  volume  of  the  air  is  so  great 
that  the  carbon  dioxide  makes  no  difference. 

M.  That  is  not  the  reason.  As  a  matter  of  fact, 
there  is  a  state  of  equilibrium  in  which  the  air  loses  as 
much  carbon  dioxide  as  it  receives. 

P.  What  becomes  of  it  then? 

M.  Plants  absorb  it.  They  decompose  it  in  such  a 
manner  that  the  carbon  remains  in  the  plant,  and  helps 
to  form  its  tissues,  while  the  oxygen  is  returned  to  the 
air  as  a  gas. 

P.  Can  plants  really  make  oxygen?  How  can  I  see 
that? 

M.  That  is  not  difficult.  We  take  a  large  glass  funnel, 
fill  it  with  fresh  green  leaves,  and  place  it  upside  down 
in  a  wide  vessel  full  of 
fresh  water.  Then  we 
sink  it  so  deep  as  to  fill  it 
completely  with  water,  and 
we  close  the  opening  with 
a  cork.  Then  we  expose  it 
to  sunlight  (Fig.  46). 

P.  Let  me  help  you  to 
lift  the  pail. 

M.  You  needn't  trouble; 
I  push  a  plate  below  the  funnel  and  lift  out  both  together; 
the  water  will  not  run  out  of  the  funnel.  When  the  sun 
shines  on  it,  you  sec  gas  bubbles  rising,  which  collect  at 
the  top. 

P.  Isn't  that  only  the  gas  which  has  been  dissolved  in 
the  water,  and  which  escapes  when  it  is  heated  (page  122)  ? 


Fig.  46. 


246  CONyERSATIOm  ON  CHEMISTRY. 

M.  No,  the  water  doesn't  grow  warm  so  quickly.  W^ 
shall  leave  it  standing  in  the  sun  till  we  have  collected 
some  cubic  centimetres  of  gas.  Then  we  will  put  the 
funnel  back  in  the  pail,  and  hold  it  so  that  the  water 
stands  at  the  same  level  inside  and  outside;  now  we  can 
take  out  the  cork,  and  test  for  oxygen  by  means  of  a 
glowing  spail. 

P.  That  is  a  beautiful  experiment.  I  shall  think  of 
plants  quite  differently  now.  What  a  lot  of  good  they 
do!  I  should  never  have  thought  it,  for,  by  breathing 
and  burning,  all  the  oxygen  in  the  air  would  be  used  up 
at  last.     Plants  give  us  it  back  again. 

M.  You  see  that  we  owe  a  debt  to  plants  because  they 
not  only  serve  as  food,  but  also  because  they  restore  us 
the  oxygen  with  which  we  bum  our  food. 

P.  I  don't  quite  understand  that.  I  eat  as  much 
meat  as  vegetables. 

M.  But  the  animals  whose  flesh  we  eat  live  upon 
plants.  We  never  eat  carnivorous  animals.  But  if  we 
did,  these  eat  graminivorous  animals,  so  that  in  the  long 
run  man  and  animals  are  nourished  by  plants. 

P.  Yes,  I  see  that.  But  if  plants  restore  the  oxygen 
to  the  air,  air  in  the  fields  and  woods  must  contain  much 
more  oxygen.  Perhaps  that  is  the  reason  that  the  air 
feels  so  fresh,  and  that  it  is  healthy  to  live  in  the  country. 

M.  No,  that  is  not  the  reason.  The  difference  between 
the  amount  of  oxygen  in  country  air  and  in  town  air  is 
very  small — it  can  hardly  be  detected. 

P.  How  is  that?  Does  it  not  contradict  what  you 
have  just  told  me? 

M,  The  air  is  in  perpetual  motion,  and  it  is  mixed  up 
so  rapidly  that  the  differences  quickly  disappear.  Even 
a  very  moderate  wind  travels  a  mile  in  five  minutes. 


THE  SUN.  247 

You  can  think  how  quickly  the  air  reaches  the  town 
from  the  wood,  and  vice  versa. 

P.  But  above  the  sea? 

M.  There  is  no  difference.  Not  only  animals,  but  also 
myriads  of  small  plants  hve  in  the  sea.  They  all  act  in 
the  same  manner,  only  they  do  not  decompose  the  carbon 
dioxide  in  the  air,  but  that  which  is  dissolved  in  the 
water,  and  they  restore  the  oxygen  in  solution.  The 
fishes  and  other  sea  animals  make  use  of  it,  for  they  too 
must  derive  their  energy  from  the  combustion  of  their 
food. 

P.  Yes,  they  breathe  through  gills.    What  are  gills  ? 

M.  They  drive  the  oxygenated  water  through  struc- 
tures which  are  permeated  with  blood-vessels  just  like 
the  lungs,  and  in  which  the  carbon  dioxide  of  their  tissues 
is  exchanged  for  oxygen. 

P.  Just  in  the  same  way  as  with  animals  that  breath'.* 
air,  except  that  water  takes  the  place  of  air. 

M.  Quite  right ;  and  there  are  still  simpler  lower  animals 
in  which  the  water  penetrates  straight  into  their  tissues. 

P.  It  all  goes  round  in  a  circle;  what  the  animals  do 
not  want  the  plants  take  up,  and  what  they  throw  out 
the  animals  use.     Does  the  same  happen  with  nitrogen  ? 

M.  Yes;  only  nitrogen,  as  I  have  told  you,  must 
always  remain  combined  (page  187). 

P.  I  remember;  and  if  the  nitrogen  becomes  free,  it  is 
again  made  to  combine  in  the  soil.  How  wonderful! 
But  tell  me  one  thing;  I  want  to  ask  you  why  the  leaves 
must  stand  in  the  sun  before  they  give  up  oxygen  ? 

M.  You  should  be  able  to  answer  that  yourself.  When 
carbon  burns  to  carbon  dioxide,  much  heat  is  liberated. 

P.  Of  course,  and  that  is  the  source  of  the  work  done 
by  machines  and  animals. 


248  CONVERSATIONS  ON  CHEMISTRY. 

M.  Then  in  order  to  decompose  the  carbon  dioxide 
again,  the  same  work  must  be  done  on  it  which  was 
Hberated  vv^hen  the  carbon  and  oxygen  combine.  Where 
do  the  plants  get  this  work? 

P.  I  haven't  thought  of  that.  You  said  something 
about  the  sun;   do  they  get  it  from  the  sun? 

M.  Of  course  they  do.  Plants  lead  a  double  life. 
On  the  one  hand  they  must  work  exactly  Uke  animals. 
They  must  pump  water,  they  must  grow  in  size,  they 
must  form  buds  and  frui'..  They  can't  make  this  work 
out  of  nothing;  they  must  take  it  from  somewhere  by 
consuming  food.  Now  they  differ  from  animals  in  this: 
they  make  their  own  food,  and  they  derive  the  necessary 
work  or  energy  from  sunlight. 

P.  You  say  that  plants  derive  their  energy  from 
food  Uke  animals.  Then  they  must  give  out  carbon 
dioxide  ? 

M.  So  they  do.  And  that  is  what  their  double  life 
consists  in.  For  the  work  that  they  carry  out  as  animals 
they  derive  the  necessary  energy  from  combustion.  But 
they  collect  this  energv  from  sunlight;  indeed  they  must 
collect  far  more  than  they  give  out  so  as  to  have  a  reserve 
for  the  dark.  And  so  they  always  evolve  carbon  dioxide ; 
but  that  can  only  be  detected  in  the  dark,  for  in  sunlight 
oxygen  is  evolved  at  the  same  time,  and  its  amount  is 
far  more  than  that  of  the  carbon  dioxide. 

P.  How  do  plants  collect  the  energy  from  the  sun  ? 

M.  We  do  not  know  much  about  that.  So  far  as  we 
know  only  green  plants  can  do  so ;  colourless  plants  like 
fungi  and  moulds  Hve  like  animals  on  plant-food;  for 
example,  rich  soil,  decomposing  vegetable  matter,  and  so 
on.  We  do  not  know  what  becomes  of  the  carbon 
dioxide  in  the  leaves  where  the  energy  is  stored  up;   we 


THE  SUN  249 

only  know  that  the  first  product  which  we  can  detect  is 
starch.  You  must  look  on  the  green  cells  of  plants  as 
little  chemical  laboratories,  in  which  are  prepared  the 
substances  which  the  plant  requires,  and  which  are 
fitted  with  arrangements  to  change  sunlight  or  the 
radiant  energy  of  the  sun  into  the  energy  of  chemical 
compounds. 

P.  Then  does  our  life  really  depend  upon  the  sun? 
I  remember  that  you  told  me  (page  180)  that  the  motion 
of  the  water  and  the  air  on  the  surface  of  the  earth  was 
caused  by  the  heat  of  the  sun.  Really  everything  which 
takes  place  upon  the  earth  appears  to  depend  upon  the 
sun. 

M.  You  are  nearly  right,  for  I  know  only  one  process 
which  does  not;  that  is  the  ebb  and  flow  of  the  tide, 
which  are  caused  by  the  attraction  of  the  moon  for  the 
sea  when  the  earth  revolves.  But  that  is  infinitesimally 
small,  compared  with  the  work  done  by  the  sun. 

P.  How  does  it  happen  that  everything  depends  on 
the  sun? 

M.  It  happens  that  the  radiation  from  the  sun  is  the 
only  source  of  energy  that  we  have  at  our  disposal.  As 
everything  that  takes  place  can  only  take  place  by  the 
expenditure  of  work  or  energy  everything  depends  on 
the  source  of  the  energy. 

P.  It  does  not  seem  so  important  to  pay  attention  to 
the  elements  being  formed  from  their  compounds  and 
the  compounds  being  decomposed  into  elements  again 
as  I  had  thought.     It  was  such  a  good  scheme. 

M.  It  is  less  important  than  the  stream  of  energy 
which  is  poured  from  the  sun  on  the  earth,  and  is  taken 
up  and  stored  by  plants  in  order  to  make  life  possible. 
You  can  make  a  picture  of  this  by  thinking  of  a  mill. 


250  CONVERSATIONS  ON  CHEMISTRY, 

The  elements  are  the  wheel  which  moves  in  a  circle  and 
continually  utilizes  the  work  of  the  falling  water.  And 
the  falling  water  represents  the  rays  of  the  sun,  without 
whose  action  the  mill  of  life  would  stop. 


Jlff^ 


M 


D2996 
Jhemistry. 


OOL  LIBRARY 


